Is There A Best Time To Exercise?

In the February, 2015 issue of Applied Metabolics,

I discussed whether the common practice of engaging in aerobic exercise on an empty stomach actually does lead to increased fat oxidation.

This is one of the many exercise concepts that bodybuilding and fitness competitors take for granted: that doing aerobics without eating first will always lead to more fat burning effects. On the surface, it makes perfect sense. The usual practice involves doing the aerobics soon after awakening, before any food is consumed. Most forms of exercise are powered mainly by stored glycogen, a long-chain carbohydrate, stored in the liver and muscles. But small amounts of glycogen are always broken down by enzymatic action in the liver to maintain blood glucose levels within a certain normal range. As such, if you haven’t consumed any food for over eight hours, such as when you sleep, your morning glycogen levels are lower than usual. The theory behind fasted aerobics is that this lower level of glycogen will allow you to tap into an alternative energy source, stored body fat, faster than usual.

While the theory makes perfect sense, as my article noted, the studies that have examined the effects of fasted aerobics, including that done in the morning prior to eating a meal, have largely come up emptier than morning glycogen stores. Conversely, eating a meal prior to morning aerobics results in greater or equal fat loss effects compared to doing the same exercise in a fasted state. There are various reasons to explain this, which I discussed in detail in that previous article. But this also brings to mind another common question: is there a preferred time to workout? That is, does training at a certain time tend to produce more gains in muscular size and strength than at other times? And is there a specific time that will lead to more efficient fat oxidation or “burning” during training?

To understand the temporal, or time-based effects of training, you need to know about about something called chronobiology


What is chronobiology, and why should I care?

Chronobiology is the science of biological rhythms. Your body has a built-in natural clock that governs various mechanisms in the body, including the release of hormones. Perhaps the most familiar example of a biological rhythm is the need for sleep. The body is programmed to sleep at certain times, mainly at night. You are set up to sleep at night by exposure to light, which affects the pineal gland in the brain. This gland converts a brain neurotransmitter, serotonin, which itself is produced from the essential amino acid, L-tryptophane, into another hormone called Melatonin.This conversion slowly occurs all day, although the melatonin release is blocked by exposure to light. When darkness begins at night, melatonin is released. It then promotes a cascade in the brain that results in sleep onset. This is a biological rhythm hard-wired into the brain and body. But this natural rhythm can be disturbed, which may inhibit or block the timed release of melatonin. An example of this is exposure to bright light at night. One way to do this is to look at a bright computer screen at hours when you would normally be asleep. This bright light exposure blunts the normal release of melatonin, and can lead to chronic insomnia. As you get older, you also tend to produce less melatonin, just as men tend to produce less testosterone and women, less estrogen. This lowered melatonin release is one reason why older people tend to have disrupted sleep.

Recent research has shown that sleep is far more vital to health and brain maintenance than previously believed. In the past, the main function of sleep was thought to be a type of clearing out of unneeded thoughts. Sleep also promoted rest and recuperation through mechanisms that included greater growth hormone release. Growth hormone release peaks at night, usually during the first 90 minutes of sleep. Since growth hormone is involved in the repair and maintenance of body tissues, it makes sense that GH levels would peak during sleep.More recent research shows that sleep also affects other hormones, including testosterone and insulin. Just one night of insufficient sleep can lower levels of testosterone by 15% in young men, who are at the peak of their testosterone production. Lack of sleep is also associated with rapid-onset insulin resistance. Insulin resistance, where insulin becomes less effective at controlling blood glucose levels, is considered the first stage of diabetes. Indeed, the most recent studies show that chronic lack of sleep is a potent risk factor for the onset of type-2 diabetes.

While this type of diabetes is usually linked to being overweight or having excess levels of body fat, the effects of insufficient sleep are so potent that even those with normal body fat levels can develop diabetes if they don’t get enough sleep

The effects and onset of sleep are a good example of how we are affected by biological rhythms or chronobiology. Chronobiology has other far-reaching effects. For example, taking drugs at certain times makes them work better because they are taken in concert with changes in daily biological rhythms. For instance, aspirin is best taken in the early morning hours for two reasons. The first is that it’s absorbed better in the morning. The second relates to the cardiovascular preventive effects of aspirin. Most heart attacks are caused by a clot in a partially obstructed coronary artery. It turns out that the clotting process in the blood peaks in the morning. This explains why most heart attacks and strokes happen during the early morning hours. Aspirin is known to reduce enzymatic reactions that are vital to the clotting mechanism. This is why doctors often advise those who are at greater risk of having a heart attack or stroke to ingest small doses of aspirin, about 81 milligrams, each day as a preventive effect against heart attacks. Since the internal clotting process peaks in the morning hours, taking a “baby”aspirin or 81 milligram dose at that time, will offer greater protection against heart attack onset by lowering the chance of an internal clot forming in a coronary artery. Since prophylactic aspirin use is also linked to cancer prevention, particularly colon cancer, ingesting it in the morning will increase the preventive effect there, too. This is an example of working with your existing biological daily or diurnal rhythms.

Testosterone also peaks in the morning, as does cortisol. Testosterone is considered an anabolic hormone, in that it promotes increases in muscular size and strength. Cortisol, which is released by the adrenal glands, is a catabolic hormone that predisposes to breakdown of muscle, especially when testosterone and growth hormone levels are lower. Both of those anabolic hormones oppose the catabolic activity of cortisol in muscle. If you are suspected of being deficient in testosterone, most physicians will have you take three testosterone blood tests spaced days apart, and they will have you take the tests between 8 and 10 a.m, which is when testosterone levels normally peak in men. You take the test three times because plasma testosterone levels can widely vary, even in those with normally consistent testosterone levels. As noted earlier, even a lack of just one night’s sleep can significantly lower resting testosterone levels. So if you have a blood test for testosterone after a night of sleep deprivation, there is a good chance that you might show a lower than normal level. But if you take it three different days, not consecutively, the chances of getting a more accurate level is increased. Some suggest that since testosterone levels peak through a biological rhythm mechanism in the early morning hours, it makes sense from a physiological point of view to use any supplemental testosterone, such as an injection or gel, at that time. The theory is that by using the testosterone then, your body will consider it a normal testosterone surge. This, in turn, may lead to less side effects that may possibly occur with testosterone usage. Again, just another example of working with your existing body rhythms.

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But what about training? Does the principle of biological rhythms or chronobiology also apply to working out? As noted, some have used the notion of training within biological rhythms to justify the idea of doing aerobics on an empty stomach as a means of increasing fat oxidation. But that particular method doesn’t seem to work–or does it? A new study suggests there may be a way that doing early morning aerobics does actually boost fat oxidation–but not in the way commonly believed.

The chronobiology of exercise

I first wrote about the chronobiology of exercise over 25 years ago in a article for a popular bodybuilding magazine. Until then, no one had even considered this aspect of training. The usual articles covered exercises, sets, repetitions, and so on, but few looked into the less obvious features of training to gain, such as how to use the effects of chronobiology to best advantage. These effects are potent, especially for a natural or drug-free bodybuilder, since they are capable of maximizing anabolic effects in muscle related to training, as well as boosting the recuperative aspects of training. Since that time, many studies have looked at this aspect of training, related to both aerobic and strength training. What they have found, however, surprisingly doesn’t differ much from what I reported a quarter century ago.

What these studies have repeatedly shown is that training becomes more efficient when core-temperature rises. The rise in core body temperature increases energy production, because enzymes involved in the conversion of food calories into energy function best within a certain temperature range. People engaged in training have long known about the importance of warming-up prior to engaging in more intensive training. The consensus to explain the benefits of a warmup is that it helps prevent injuries. There is some truth to that, since muscles and connective tissue that have not been directly worked are in a sense, cold and stiffer. The act of warming up delivers blood to the area and increases the local temperature of the tissues. More importantly from the point of view of muscle gains, is that warming up boosts a process in muscle technically known as actin-myosin crossbridging.Actin and myosin are the proteins that comprise contractile muscle fibers, and muscle contracts when these proteins slide over each other, which is the “crossbridging” part.

This is a very simplified way of explaining muscular contraction, but the point is that muscle functions more efficiently at a higher temperature up to a point. It’s similar to the engine in a car. At normal operating temperature, the fuel mix delivered to the engine is optimal, and the engine works at higher efficiency. But if the engine gets excessively hot, it cannot operate correctly, and could fail. The same is true for muscle tissue. At optimal temperatures, most often in the afternoon hours, muscle functions peak, and the enzymes that produce energy in muscle are working at high efficiency,too.But when the ambient temperature gets too hot, energy processes slow down in the muscle, as the emphasis is changed from work output of the muscle to cooling processes. Those people who train in heavy clothes during hot temperatures are working against themselves, producing premature fatigue of the muscle. They seem to think that heavy sweating will cause them to lose body fat, which is nonsensical.

Most studies have shown that afternoon and early evening training produces better results than training early in the morning. When you exercise later in the day, you are working within your own circadian body rhythms. Connective tissue, such as tendons and ligaments, are stiffer in the morning, and subject to increased risk of injury. Those who train very early, such as at 4 a.m, may feel the effects of sleep inertia, which is a feeling that you’re still not awake. I experienced this effect myself years ago while training at the original Gold’s gym in Venice, the same gym where Arnold Schwarzenegger and other great bodybuilders of the era also trained. A friend convinced me to try training as soon as the gym opened, which was at 5 a.m. At the time, most of my training was usually done in the late afternoon. I would go the gym right after attending college. I will never forget those early morning workouts, since I had the surrealistic experience of being in the gym, yet not being there. Although this was during the era of psychedelic drugs, such as LSD and mescaline, I had never touched any of that stuff. Yet, my workouts seemed to be in a dreamlike state. I felt no muscle pump at all, despite extensive warm up for every muscle that I trained. When I told my friend about this, he told me to give it time. “You will get used to it, and once you do, you will never want to go back to training later in the day, since this way, you get your training out of the way early. You have the whole day to do other things,”he said. I followed his advice, and continued to train in the early morning hours for 2 weeks. But each workout felt the same: nothing, no pump, no feeling, nothing.

From a physiological point of view, it’s hard to understand why I never got anything out of early morning training. One reason could be that my anabolic hormone release, mainly testosterone, didn’t occur until after the workout, about 8 a.m. Indeed, many of the recommendations given about training at specific times, such as morning or evening, often mention how testosterone peaks in the morning, and has another peak in the late afternoon and early evening hours. In fact, since luteinizing hormone (LH), which governs testosterone release in the body, is released in pulsitile fashion (short bursts) throughout the day, testosterone goes up and down all day, gradually declining later at night. On the other hand, cortisol levels do peak during the early morning hours, which happened to coincide with the precise time that I trained at Gold’s. The significance of this is that higher levels of cortisol during training have been shown to exert an inhibitory effect on the neuromuscular system. In short, it can interfere with the complete expression of muscular coordination. This would explain my “dead” feeling when I trained during those early morning hours.

Warming up also tends to boost the use of carbohydrates as an energy source. Why is this important? Anaerobic exercise, such as typical bodybuilding workouts, are powered by glycogen, a form of carbohydrate, that is stored in muscles, and secondarily by circulating blood glucose. If you can use these energy sources more efficiently, you will be able to train harder and more intensely. Training in the morning without adequate warm-up would blunt the complete use of these energy sources.The best way to warm up in this instance, would be to do a few lighter sets featuring higher reps, about 15 to 20, prior to doing the same exercise with heavier weights. This will send the signal to start the internal metabolic fuel process, as well as boost the activity of the actin-myosin crossbridges that constitute muscular contraction. One study showed that if those engaged in early morning exercise added 20 minutes of active warm up prior to training, they were able to increase their body core temperature to levels comparable with afternoon training.

Still, most studies show that training in the afternoon or early evening is usually more productive in terms of muscular and strength gains compared to early morning training. The muscles seem to just function better later in the day. One suggested reason for this has to do with the way that muscles handle calcium. Calcium is involved in muscular contraction, and is released from a portion of the muscle called the sarcoplasmic reticulum. Other studies suggest that levels of inorganic phosphate also increase later in the day, and that this also influences muscle function by interacting with the actin-myosin crossbridge system.

As noted, the various anabolic and catabolic hormones in the body, including testosterone, growth hormone, and cortisol, tend to fluctuate throughout the day. Early studies that examined what was the best time to train, often suggested that training coincide with the maximum natural release of anabolic hormones, while avoiding times when cortisol would dominate. Since high levels of cortisol do appear to interfere with complete muscular expression during training, either avoiding training when cortisol is high (such as the early morning) or using supplements, such as phosphatidylserine (PS) (300 milligrams) that may partially blunt cortisol activity may be prudent. The situation, however, with anabolic hormones is less clear. Recent studies show that the temporary rise of anabolic hormones, such as testosterone and growth hormone, following training exerts little or no true anabolic effect. This relates to the short-term presence of these hormones, which only peak for an hour or so following training, then rapidly drop to baseline. For the anabolic hormones to produce muscle gains, there must be sustained levels of the hormones in the blood. Since using drugs such as anabolic steroids do exactly that, this explains why such drugs do reliably promote muscular size and strength gains. The point here is that tailoring your workouts in an effort to train when anabolic hormones are are their highest peak will likely not have any significant effect on muscle gains.


So what does this all add up to? Some people are more comfortable training in the early morning. Others prefer to train later, in the afternoon or evening. The important thing is that you need to increase your core temperature, no matter what time you train. If you opt to train in the early morning hours, it would make sense to do more extended warm-ups. This might mean doing two lighter, high rep sets prior to doing your heavier work on the same exercise. Training later in the day may require only one warm-up set to fully prepare your muscles for the more intense work to come. In truth, however, the ideal training time in relation to anaerobic training, such as most bodybuilding workouts, is that the best time is when you feel most comfortable, and when your energy levels are high. But the situation does change for purposes of fat oxidation. If maximum fat loss is your goal, recent research shows that training at certain times does affect your rate of fat oxidation.

Optimal training time for fat oxidation

While the optimal training time for anaerobic exercise appears to be in the late afternoon or early evening, but in reality doesn’t make much difference as long as you increase body core temperature through engaging in an adequate warm-up, the same may not be true for aerobic training. Two recently published studies came to opposite conclusions about the best time to train for purposes of promoting increased body fat oxidation. The first study noted that exercise after eating has little or no effect on 24-hour fat oxidation. In simple terms, if you eat a meal, then exercise, you may oxidize some fat during the exercise session, but it stops when the workout ends. For fat-loss efforts to be effective, it would make sense to do something that might promote all-day fat oxidation, even into the evening hours. The study involved 10 young, nonobese men, whose calorie usage was measured by having them stay in a closed metabolic chamber, where every calorie used could be accurately measured. They were observed over a 24-hour period. A few did no exercise in the chamber, but other subjects did an hour of exercise at an intensity level of 50% of maximum oxygen intake under three conditions: before breakfast; after lunch; or after dinner.All the men in the study consumed the same amount of daily caloric intake.

The results showed that fat oxidation in the men was only increased for 24 hours when the men engaged in an hour of exercise before eating breakfast. Exercising following the afternoon and evening meals did not affect 24-hour fat oxidation in the men. The study authors ascribed this increased fat oxidation from the fasted aerobics to a transient deficit in energy and glycogen made even more acute by the exercise done in the morning prior to eating. What they are saying is that exercising in the morning prior to eating causes such a deficit in energy and glycogen that the body upgrades fat oxidation all day to make up for the deficit. This, however, is not in agreement with the majority of prior studies that show exercising prior to breakfast does not lead to increased fat oxidation, and in fact, eating before engaging in aerobic exercise produces better effects than fasting aerobics in terms of fat oxidation. I covered these prior studies in detail in the February, 2015 issue of Applied Metabolics.


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This study wasn’t concerned about how much fat was burned by doing morning fasting aerobics, but rather how it affected the use of fat for energy over a longer time,that is, 24 hours.The study authors included subjects who didn’t exercise to compare the use of fat as an energy source in those who exercised to sedentary people. The diet consumed by both the sedentary controls and the active exercisers was the same, consisting of 15% protein;25% fat; and 60% carbohydrate. The researchers did find that those who did the morning fasted aerobics burned more fat during the exercise. This appears to confirm the prevailing belief of many bodybuilders, who think that doing aerobics in a fasted state in the morning leads to considerably greater fat oxidation.Training in the evening tended to burn the most carbohydrates, followed by afternoon then morning. In short, the use of fuel shifted more towards carbs if the exercise was done after meals later in the day compared to doing the same exercise in a fasted state in the morning.

The study authors found that glycogen depletion following the morning fasted aerobic session was equal to 18% of whole body glycogen storage. Again, they are saying that 24-hour fat oxidation correlates to the extent of depleted glycogen reserves. There is some truth to this. Although carbohydrates are often thought of as the most fattening of macronutrients (the others are protein and fat), body fat isn’t synthesized from consumed carbohydrates until muscle glycogen reserves are completely filled. In simple terms, as long as your glycogen stores aren’t filled, ingested carbs will be diverted for use in glycogen synthesis. Of course, the main way that glycogen stores become depleted is through exercise. For those who are sedentary, glycogen stores are usually filled unless they are consuming either few calories or are on a low carbohydrate diet. With the usual dietary suggestion to consume 60% of total daily calories are as carbs, most people who don’t exercise will have replete glycogen stores. As such, if they consume excess carbohydrates that exceed energy use, those ingested carbs will be converted into body fat. The fact that obesity is now epidemic shows how easily this happens, although most people who are obese get that way from eating a combination of excess fat and carbs. Consuming carbs or fat alone makes it far more difficult to add body fat. If you focus only on carbs, and stay active, your carbs will be used mainly to replace depleted glycogen stores. But if you also consume excess fat with the carbs, most of the ingested carbs will be shuttled to fat synthesis. Excess fat intake when ingested with excess carb intake leads to a relative insulin resistance that interferes with the way carbs are usually used for muscle glycogen synthesis processes. If you consume a lot of fat without carbs, as is common with low carb diets, the body will switch from using mainly carbs as a fuel source to using ingested and stored fat. As such, that fat you’re eating will be oxidized for energy rather than stored as body fat.

When you deplete glycogen, you also activate body signaling factors that detect the depleted energy stores in the body (glycogen). One example of this is that an energy-sensing protein is activated called AMPK. AMPK is always activated under low energy or depleted glycogen conditions. It’s the body’s way of tapping into alternative fuel stores, including stored fat. When you do aerobic exercise to the point of depleting some glycogen, AMPK is activated in the working muscle. AMPK then activates a cascade in muscle that results in greater use of intramuscular fat and fatty acids circulating in the blood. In short, AMPK turns on the fat oxidation process in muscle. Low glycogen stores in muscle also activate genes that, when activated, increase the activity of intramuscular enzymes involved in upgraded fat oxidation. These processes explain how 24-hour fat oxidation could be increased if enough glycogen is depleted through exercise or training in a fasted state. Indeed, if you want to obtain the maximum fat loss effect from an intermittent fasting regime, you would be well advised to do a aerobic exercise session during the fast. The choice is up to you as to what type of aerobics to use for this purpose, but interval training would likely be the most efficient, since it would lead to greater glycogen depletion and thus set into motion the AMPK/fat loss genes effect.You need do no more than 20 to 30 minutes of interval aerobics during an intermittent fast to obtain best results. AMPK does a lot of other things, and may be a key factor in maintaining health and fitness as you age. This will be discussed in an upcoming article in Applied Metabolics.

Conversely, ingesting carbohydrates during your workout, or shortly prior to doing aerobics will blunt any fat oxidation effect induced by the exercise. Note that the AMPK release only occurs during low energy or low glycogen conditions. If you ingest the most readily used fuel source–carbohydrates–your body has no reason to tap into fat stores, since you are supplying the fuel needed to power the exercise.

One of the dangers of engaging in fasted aerobics is that you could tap into muscle protein stores as a source of energy, especially of the exercise exceeds 30 minutes. In this study, however, the men showed no increase in protein oxidation over the measured 24-hour period. The study authors also note that this study only involved a 24-hour period. They have no idea of whether the increased fat oxidation shown in this study would continue long-term. They suggest, however, that it would not because of body compensation effects whereby the body would switch to using carb as fuel rather than fat. This could explain the failure of fasted aerobics to promote greater fat oxidation that was apparent in most previous studies. So it may be that the increased fat oxidation promoted by fasted aerobics done prior to breakfast is only a shortterm effect. But another study found that doing early morning aerobics was not the best way to to oxidize increased amounts of body fat. In fact, that study came to an opposite conclusion.

This other study focused on 14 healthy man, average age, 24, who did endurance exercise or aerobics in the morning on one day, and in the evening on the next day. They then did nothing for a week, then repeated the same exercise sequence, first morning, then evening exercise. The exercise consisted of walking at a pace equal to 60% of maximal oxygen intake. You will note that in this study,as in the previously discussed study, the subjects engaged in low intensity aerobics as measured by maximal oxygen intake. The reason for this is simple: You burn more fat when doing lower intensity aerobics. As the intensity level increases, your body switches to a more readily available fuel source, mainly circulating glucose and glycogen stored in muscles. Interval training burns fat because of the greater glycogen usage involved, which boosts fat oxidation following exercise.

The mechanism that causes the release of fat stored in fat cells is an enhanced release of catecholamines, which are hormones, such as epinephrine and norepinephrine. Exercise promotes the release of catecholamines to help provide fuel for the exercise. When catecholamines react with adrenergic cell receptors in fat cells, a cascade is initiated that results in enzymes degrading the fat stored in the fat cells or triglyceride, into its components, free fatty acids and glycerol. The free fatty acids then circulate in the blood and can be oxidized during the exercise. Most of the current “fat burner” supplements work by providing substances that may upgrade the release of catecholamines, such as caffeine. As such, when used in conjunction with exercise, especially that which features a greater intake of oxygen (aerobics), the supplements may add to the fat oxidizing effect induced by exercise.

What this other new study showed was that exercising in the evening resulted in a much greater release of catecholamines compared to exercising in the morning doing the same exercise at the same level of intensity. This was shown by the much greater level of free fatty acids that appeared in the blood when working out in the evening compared to morning sessions. The greater release of catecholamines in the evening also activated a cytokine, which is a protein that works as a signaling factor in promoting complete immune response. This particular cytokine, Interleukin-6, can have either catabolic or anabolic effects in muscle. When secreted in greater amounts at rest, it promotes excess systemic inflammation, which is associated with muscle catabolism or breakdown. Interluekin-6 is thought to be a major player in the onset of sarcopenia, a general term for the loss of muscle mass with age. But during exercise, IL-6 within muscle acts completely differently. It promotes increased muscle protein synthesis and helps to activate muscle stem cells called satellite cells that are involved in muscle repair and growth. IL-6 in concert with catecholamines, promotes the release and oxidation of stored body fat. This study of young men engaged in aerobic exercise clearly showed that exercising in the evening led to a far more pronounced release of both catecholamines and IL-6, which in turn, led to higher levels of free fatty acids in the blood. IL-6 in muscle is produced during intense muscle contractions.

So we have two studies that came to opposing conclusions. Since the first study only measured 24-hour oxidation for one day, and there was no evidence presented that this effect would continue indefinitely, coupled with the preponderance of previously published studies showing that fasted aerobics offered no particular advantage in terms of increased fat oxidation, I would suggest that if increased fat oxidation is your goal, training at night would be the better choice. Apparently, the circadian rhythms in the body that favor increased fat oxidation peak during the evening hours, and training at that time would take advantage of this natural biological rhythm.

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About Jerry Brainum

jerrybrainumAllow me to introduce myself. My name is Jerry Brainum, and anyone who has read bodybuilding magazines over the past 35 years will likely recognize my name. I’ve had over 5,000 magazine articles published under my own name and various pen names. I was the science editor of Muscle and Fitnessmagazine for 10 years; editor-at-large at Flex magazine for 8 years. I’ve also written for Men’s Fitnessmagazine; Sports Fitness magazine; Muscular Development magazine; Muscle Mag International, and was a regular contributor to Ironman magazine for 28 years.  Most recently, I was the science editor of Planet Muscle magazine. I’ve also written for many magazines outside of bodybuilding, including Penthouse andLet’s Live magazines. Read More


Teo, W, et al. Circadian rhythms in exercise performance: Implications for hormonal and muscular adaptation. J Sports Sci Med 2011;10:600-606.

Seo, DY, et al. Morning and evening exercise. Integrative Med Res 2103;2:139-44

Iwayama, K, et al. Exercise increases 24-hour fat oxidation only when it is performed before breakfast. Ebiomedicine 2015: in press.

Ki-Kim, H, et al. Effects of acute endurance exercise performed in the morning and evening on inflammatory cytokine and metabolic hormone responses. Plus One , Sept 9, 2015.