Sleep is a basic human need. For athletes and those vying for peak physical performance, adequate sleep is one of the most essential factors in achieving that goal. Sleep may be the most important aspect of recovery due to its vital role in tissue growth and repair. It also has roles in neuromuscular performance, cognitive function and memory, immune function, and emotional wellbeing.(1)
The detrimental effects of sleep deprivation include cognitive slowing, memory impairment, decreased vigilance and shortened attention. Its effects on physical performance impact fatigue and recovery processes, and result in a decline in the ability to perform maximal aerobic and anaerobic exercise.
Growth hormone (GH)- also known as somatotropin- is a naturally occurring hormone released by the anterior pituitary gland in the brain. It has a number of different actions within the body. Growth hormone is vital for normal growth in children, and levels increase around the time of puberty, peaking during the growth spurt. (2)
Growth hormone is essential for the growth and repair of muscles. It stimulates protein synthesis and increases fat breakdown to provide energy for tissue growth. It opposes the actions of insulin. It acts to increase the retention of nitrogen, which is a key component of protein. Studies have shown that GH increases muscle cell mass and number, and causes increases in muscle nitric oxide release, leading to vasodilation and increased blood flow. There is some evidence that growth hormone stimulates nerve regeneration and muscle preservation following injury. (3) Adults with low GH levels have increased fat mass and reduced lean body mass.
GH is released in a pulsatile manner and it’s release is stimulated by a number of different factors, by far the most important of which is sleep. Release is also stimulated by lowering blood sugar, the presence of certain amino acids and by exercise. Other factors affecting its release are other hormones such as ghrelin and prolactin.
GH levels increase from 20 to 90 minutes from the onset of sleep and peak between 40 and 165 minutes, with a mean peak of around 70 minutes. Levels remain elevated for 1.5 to 3.5 hours then return to baseline. In women, there is some evidence of further peaks which are lower than the first. Growth hormone is released during deep sleep (stages 3 and 4) rather than during REM sleep.
If the onset of sleep is delayed, the peak of GH is delayed and is lower than in non-delayed sleep. Short interruptions (2 to 5 minutes) to deep sleep cause lower levels of growth hormone release, however a prolonged interruption of 2 hours may cause a second peak after falling asleep the second time.
It was initially thought that this increase in GH during sleep may be related to blood sugar levels. As time since the last meal increases overnight, blood sugar levels drop and it was thought this may trigger growth hormone release to increase fat breakdown and free fatty acid levels in the blood. As such, there were concerns that eating carbohydrates late at night may reduce growth hormone production and therefore reduce muscle growth and repair. There is however no correlation seen between blood sugar levels and the GH peak during sleep, and the slow rise in free fatty acids during sleep appears unrelated to GH levels.
When energy expenditure increases during the day, the blood levels of growth hormone rise during the following night, but when an athlete loses deep sleep, these levels fall significantly. (4)
Growth hormone levels are higher in women than men and decrease with age in both sexes. They peak at puberty, with the surges associated with deep sleep that is particularly prominent at that age. There is some evidence that GH levels are associated with estradiol, but not testosterone levels. This would imply that growth hormone levels in women would be at their highest during the periovulatory phase of the menstrual cycle, when oestradiol is at its peak, however there is mixed evidence for this. (5, 6)
It is vital that athletes understand the importance of prioritising sleep. A 2005 study demonstrated that when athletes were asked to sleep the maximum amount possible vs a habitual length sleep, there were significant improvements in performance, mood and a decrease in fatigue. (7)
Athletes should understand how sleep affects performance and recovery, so that they are motivated to identify which factors affect their sleep quality. They can then use this information to develop optimal sleeping habits which will help them perform at the best of their ability.