Good sleep: train early or burn the midnight oil?

In recent years, scientists have established that sleep and sport/exercise performance have a strong relationship, each mutually influencing the other, both positively or negatively. Get your sleep health right and performance is likely to be enhanced. However broken or insufficient sleep is not good for performance. In terms of physiological recovery, sleep is vital – not least because a number of hormonal responses take place in the lead up to and during sleep.

One important hormone relating to athletic recovery is growth hormone. Growth hormone is necessary for body restoration, and plays an important role in muscle growth and repair^(1,2). Muscle growth, repair, and bone building are vital for athletic recovery following strenuous training and competition; it has been reported that 95% of the daily production of growth hormone is released from the pituitary gland in the endocrine system during non-rapid eye movement sleep (NREM - ie deep sleep). Therefore NREM sleep is considered the time in which the body actively repairs and restores itself^(3,4).

Sleep quality matters

Given the physiological and biochemical impacts that sleep helps facilitate in the human body, it follows that anything that impairs the length or quality of sleep in an athlete can also be detrimental to maximizing the speed of recovery, and therefore subsequent performance – and research has established that this is indeed the case^(5,6). In particular, sleep deprivation is associated with higher rating of perceived effort (RPE) values, potentially leading to reduced performance, particularly in endurance events. In short, when you’re sleep deprived, you will feel like you’re working harder to sustain a given workload compared to when sleep amounts have been adequate.

Training timing and sleep quality

Since optimum recovery from training requires adequate sleep quality and quantity, and that sleep quality (and to a lesser extent quantity) can be in turn affected by training timing, the question arises of how best to time training sessions to maximize sleep quality, and therefore ensure more rapid recovery. This is a topic that a number of studies have tried to answer.

One US study examined the relationship between sleep quality and time/intensity of exercise undertaken that day⁽⁷⁾. To do this, researchers looked at the sleeping and exercise habits of 1000 adults aged 23-60 years from a wide variety of US geographical regions. In particular, the researchers asked the subjects to report their sleep quality, total sleep time, sleep latency (how long it took them to fall asleep) and how often they woke without feeling refreshed in the morning. The timing of that day’s exercise was also recorded and split into one of three categories:

·       More than 8 hours before bedtime

·       Between 4-8 hours before bedtime

·       Less than 4 hour before bedtime

The results showed that training late in the evening had no detrimental effect on sleep quality; compared to those who exercised more than four hours before bedtime, those who exercised nearer to bedtime reported no difference in their sleep quality, total sleeping time, sleep latency or how often they woke without feeling refreshed in the morning. This was the case regardless of whether the ‘late trainers’ had exercised at moderate intensity or vigorously prior to bedtime. Having said that, the highest scores for sleep quality were for those who had exercised vigorously in the morning – these subjects had the lowest likelihood of waking up in the night and not feeling refreshed in the morning.

These findings tie in somewhat with research carried out the previous year by Chinese researchers who examined the effect of evening exercise in 5086 students⁽⁸⁾. This study found that moderate-intensity exercise such as steady-state jogging and cycling performed late in the evening had no negative impact on the students’ sleep quality. However, unlike the US study above, very intense exercise performed later in the evening DID seem to harm sleep quality.

Training timing and sleep quality in highly-trained endurance athletes

Although both of the above studies investigated sleep timings in healthy, recreationally active adults, the findings come with a caveat: while the subjects were exercising regularly, they were not classed as athletes. This is important because for any given workload, the body of a highly trained athlete will be under much less relative physiological stress than would be experienced by a relatively untrained person. Also, lower volumes of low-moderate intensity exercise do not produce the same kind of hormonal impacts that higher exercise volumes and intensities (more typically used by athletes in training) do.

In 2019, a study by Spanish scientists addressed this concern by investigating ultra-endurance runners, whose training volumes and intensities are probably much more representative of most endurance athletes who train seriously⁽⁹⁾. In this study, the scientists analysed the effect of the intensity and the hour of the training session on sleep quality and ‘cardiac autonomic activity’ (cardiac autonomic activity refers to the subconscious nervous control of the cardiovascular system, and includes heart rate variability, which is often measured as a determinant of physiological recovery after exercise).

All the runners underwent four trials (separated by 72 hours) after which sleep quality and cardiac activity were monitored over the course of the following night. These trials were as follows:

·       Morning training at moderate intensity

·       Morning training at vigorous intensity

·       Evening training at moderate intensity

·       Evening training at vigorous intensity

As well as subjective sleep quality and nocturnal cardiac autonomic activity, the researchers also recorded the runners’ sleep efficiency using sleep actigraphs. Sleep efficiency (see figure 2) is a measure of how rapid sleep onset is, and whether sleep is interrupted by waking periods. A high sleep efficiency means that someone falls asleep very quickly after retiring and experiences minimal periods of wakefulness during the sleep period.

Figure 2: Sleep efficiency