Is sleep really that important?

How important is sleep?

Sleep and circadian rhythms seem to regulate a variety of bodily processes. Sleep deprivation disrupts these processes.

Fat and obesity

Epidemiological research

Several studies have been conducted on the topic of sleep and obesity. Less sleep is usually associated with more body fat. Furthermore, sleep is associated with weight gain even over a longer, five-year period.

When we exclude possible confounding factors, this relationship goes beyond the genetic side of things and seems to go beyond demographic, lifestyle, occupational and health factors.

Although the research does not provide clear conclusions, there does seem to be an ongoing relationship between lack of sleep and more fat. This relationship persists even after eliminating the most obvious confounding factors. However, epidemiological research tends to focus only on sleep duration.

Sleep deprivation

During deliberate caloric restriction (dieting), a 3-hour reduction in sleep (from 8.5 to 5.5) appears to be related to an undesirable nutrient partitioning that causes us to lose more net muscle mass than fat, measured against caloric restriction when we get enough sleep.

Sleep restriction can adversely affect nutrient partitioning in weight loss.

Sleep restriction acutely increases hunger and may be more significant when it occurs with reduced caloric intake. In healthy women, this has been reported as a roughly 20% increase in voluntary energy intake. Their weight increased by 0.4 kg over 4 days.

Sleep deprivation has been shown to increase circulating leptin levels. (29%)

Sleep deprivation can lead to higher fat gain in the long term (perhaps due to hunger, which is definitely increased), but it can also interfere with weight loss in the short term by changing the ratio of lean fat to muscle.

Cognitive performance

Sleep restriction leads to a neural pattern sometimes seen in depression, and well-being also seems to be linked to sleep. Sleep restriction disrupts higher thought processes such as problem solving.

Disrupted sleep impairs cognitive performance.

Insulin sensitivity

Epidemiological research

There is a relationship between abnormal sleep patterns and metabolic syndrome. Less sleep and irregular or disturbed sleep is associated with comorbidities of metabolic syndrome (insulin resistance, hypertension, obesity). Insulin resistance to diabetes is seen in persons with poor sleep habits. Although meta-analyses and other tests show increased risk for both shortened(5-6 hours) and prolonged sleep(8-9 hours).

Both insufficient and excessive sleep is associated with insulin resistance and increased risk of diabetes. And they are essentially equivalent. Those sleeping 7-8 hours seem to be at the lowest risk.

Intervention studies

In healthy, young individuals, acute sleep deprivation can disrupt insulin signaling in isolated adipocytes. This can be seen after three weeks with 90 minutes less sleep or after five days of more substantial intervention ( 60 percent of time taken away). This insulin resistance was measured to be 20+/-24% or 11+/-5.5% after several days of sleep restriction of 4 hours, in healthy men.

Studies noticeably restricting sleep to a single day (4 hours of sleep) also confirm insulin resistance in healthy subjects.

Studies that limit sleep to as little as 2 hours per day can induce insulin resistance in healthy subjects in as little as one week. 4 or less hours of sleep will get you this condition in a single night.

Androgens

In older men, sleep is the sole determinant of morning total and free testosterone levels, and during aging, testosterone decline is also associated with sleep patterns. However, one study did not find an association between sleep and total serum testosterone levels.

A study comparing chronotypes, or bodily functions during the day ("morning/evening" people), found that chronotype was associated with testosterone levels, not with the total amount of time slept. Evening people typically had more testosterone. This shows that earlier studies showing a relationship between evening testosterone levels and time slept may have been obscured by diurnal fluctuations in testosterone, and testosterone levels may ultimately be independent of the amount of sleep.

In general, sleep appears to be related to testosterone levels. The strength of the relationship is not substantial, but studies acknowledge it. One study says that the issue may be more on chronotype overall than on amount of sleep. However, these factors may be related.

One study showed a 10.4% drop in testosterone in young men sleeping normally 8 hours a day when this time was reduced by 3 hours after five days. Another, equally small, study showed a 30.4% reduction in testosterone levels with sleep restriction, which was accompanied by a reduction in DHT (26.4%) and androstenedione (32.6%), despite the odd fluctuation in FSH between subjects. One study limiting sleep by 60% (the normal 10 hours was reduced to 4 for five days) saw a trend towards a reduction in testosterone levels, but did not achieve statistically significant results, despite an increase in SHBG.

This reduction in androgen hormone levels occurs thus at 24 hours of sleep restriction, but does not rise further beyond this time limit.

One study that completely banned sleep (33 hours) found an acute reduction in testosterone levels, along with less reactive aggression.

Androgen hormone levels (testosterone is the most commonly measured) are reduced by 10-30% by daily sleep restriction, and one day of sleep deprivation also reduces them.

Cortisol

Cortisol mediates waking and exhibits a predictable circadian rhythm, with high levels in the morning and low levels at bedtime.

Some studies have not found a relationship between mild sleep restriction and cortisol levels, but many have, and even after one night or five days the range was 51+/-8%.

Studies showing an increase in cortisol levels usually measure the entire day and may show an increase due to elevated evening levels caused by lack of sleep. Morning cortisol levels tend to be even lower because lack of sleep disrupts its normal rhythm. Daytime levels rise slightly.

Cortisol usually pulses, and its levels are higher in the morning and lower at night. Shortened sleep disrupts this and causes this pulsation to straighten out, but the total daily cortisol level rises slightly.

Thyroid hormones

Sleep deprivation increases these hormones in healthy women T3 (19%) and T4 (10%)). However, other studies do not confirm this with 33 hours of no sleep or chronic sleep deprivation. One study measuring the hormone TSH found an increase in it with acute sleep deprivation, but this was not confirmed with chronic conditions.

Absolute sleep deprivation for one day or sleep reduction for multiple days has no demonstrable effect on thyroid hormone levels.

Diet-induced thermogenesis can be easily suppressed by sleep deprivation, whereas cold-induced thermogenesis appears unaffected. In mice, sleep deprivation even promotes thermogenesis (and they lose weight despite increased food intake). Symptoms of sleep deprivation in mice are as follows: overeating, weight loss, increased energy expenditure, increased plasma catecholamines, hypothyroidism, decreased body temperature and impaired physical appearance.

Studies measuring metabolic rate or total energy expenditure cannot find a difference between normal sleep and sleep deprivation. Rather, an increase in metabolic rate will be due to increased physical activity.

Sleep deprivation does not slow metabolism. There is even evidence to the contrary (i.e. in mice - is it the lack of sleep itself, or increased physical activity).

Growth hormone

The major growth hormone washout comes shortly after falling asleep and has to do with slow-wave and delta-wave sleep. This washout is responsible for about 50% of the daily GH washout in young, healthy men. The relationship between GH washout and slow-wave sleep is not always clear, and some authors think that this phase of sleep merely coordinates the washout.

The total amount of GH leached appears to be higher in young individuals and women than in older individuals and men, respectively; in women, this is due to higher diurnal levels, which are positively influenced by estrogen.

Sleep brings us the highest daily dose of growth hormone. In young people, this dose equals about half of the total daily dose.

Studies using sleep-deprived individuals noting a decrease in growth hormone levels (no sleep, no major washout) also note that total daily GH production increases (but not enough to compensate). This may be due to chronic sleep deprivation (6 nights with 4 hours of sleep) causing a predictable, biphasic GH washout. One study even noted that night workers with little sleep washout (16.8+/-3.3%) have additional during the day to compensate.

It seems that when sleep GH washout is disrupted, the body compensates during the day, and thus the total daily dose of GH washout remains without major changes.

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Source:www.examine.com
Author: Examine.com team
Translated by Jaromír Uhlíř (Cha Cha)
Original article title: How important is sleep?