HOW IMPORTANT IS SLEEP?
Sleep is incredibly important, and can be considered crucial alongside diet and exercise. Proper sleep habits help sustain many biological processes, and bad sleep can cause these processes to be sub-optimal or even malfunction.
Fat Mass and Obesity
Epidemiological (Survey) Research
Several studies have been conducted looking at the correlation (degree of association) between body fat and sleep. There appears to be an inverse correlation (less sleep nightly being associated with more body fat) that is further associated with more fat mass gain over a period of 5 years
Although correlation research is not conclusive, there appears to be a persistent relationship between less sleep time and greater fat mass.
During intentional caloric restriction (fat loss diets), it appears that a reduction of sleep by 3 hours (8.5 to 5.5) is associated with an unfavourable nutrient partitioning effect, making more weight loss come from lean mass rather than fat mass relative to a rested control.
Sleep deprivation may adversely affect nutrient partitioning during weight loss
At least acutely, sleep deprivation appears to increase hunger and may be more significant when the sleep deprivation coexists with a reduced caloric intake. In otherwise healthy women, this has been quantified at around a 20% increase in voluntary energy intake (and a slight increase in body weight of 0.4kg over 4 days).
Sleep deprivation, over time, may lead to higher fat mass gains (possibly secondary to hunger, which is reliably increased) although even short term sleep deprivation appears to hinder fat loss attempts via reducing the percentage of weight loss that is fat mass
Restriction of sleep produces a neural sleep wave pattern that is sometimes observed in depression, and well-being appears to be related to sleep as well. A reduction in sleep reduces higher levels of cognition such as problem solving.
Impaired sleep is associated with impaired cognitive function.
Sleep time appears to be an individual predicting factor for both total and free testosterone in the morning in older men, and during the process of aging the decline in testosterone (associated with aging) is further associated with perturbed sleep patterns.
In general, sleep appears to be somewhat associated with testosterone levels. The strength of the correlation is not remarkable, but studies have at least noted some form of relationship. One study suggests that this may be more indicative of chronotype than overall sleep time, however, with those two factors being correlated but independent.
One study (measuring testosterone for one day during waking hours) in young male subjects sleeping 8 hours routinely that cut sleep by 3 hours for a period of 5 days reduced testosterone by an average 10.4% relative to rested control, suggesting that acute sleep deprivation is able to influence testosterone levels.
A single night without sleep is enough to decrease androgen production, and moderate daily sleep deprivation reliably reduces androgen levels (Testosterone levels being the most frequently measured) by some 10–30%.
Sleep deprivation in otherwise healthy females has been noted to increase thyroid hormones T3 (19%) and T4 (10%).
Practical sleep deprivation either absolutely for one day or a reduction over a few days does not have consistent evidence for its effects on thyroid hormone levels
Studies that measure metabolic rate or total energy expenditure fail to find significant differences between normal sleep patterns and deprivation or possibly an overall increase in metabolic rate that is due to more spontaneous physical activity (ie. movement).
Metabolic rate is not reduced with sleep deprivation, and some evidence suggests that it is actually increased with sleep deprivation.
Overall growth hormone secretion appears to be greater in youth and in women relative to older individuals and men, respectively; the increase seen in women is due to higher daytime levels being positively influenced by estrogen.
Sleep mediates the largest daily spike of growth hormone, which in young persons accounts for approximately half of daily exposure.
Studies using sleep deprived persons that note a decline in growth hormone (due to lack of sleep) note that daily GH production increases, but only enough to approximately compensate.
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