Organ systems during sleep

Sleep Disorders

Glossary

The functions of many organ systems are linked to the sleep cycle.

Endocrine system

Most hormone secretion is controlled by the circadian clock or in response to physical events. Sleep is one of the events that modify the timing of secretion for certain hormones. Many hormones are secreted into the blood during sleep. For example, scientists believe that the release of growth hormone is related in part to repair processes that occur during sleep. Follicle stimulating hormone and luteinizing hormone, which are involved in maturational and reproductive processes, are among the hormones released during sleep. In fact, the sleep-dependent release of luteinizing hormone is thought to be the event that initiates puberty. Other hormones, such as thyroid-stimulating hormone, are released prior to sleep.

Both sleep and circadian effects interact to produce the overall rhythmic pattern of the pituitary and pituitary-dependent hormones. Some of the 24-h hormonal rhythms depend on the circadian clock (ACTH, cortisol and melatonin), or are sleep related (prolactin and TSH). GH secretion is influenced by the first slow wave sleep (SWS) episode at the beginning of the night. Pulses of prolactin and GH are positively linked to increases in delta wave activity, i.e. deepest phases of sleep, occurring primarily during the first third of the night. Pulses of TSH and cortisol are related to superficial phases of sleep. As a result of the consolidation of the sleep period, the wake-sleep transition is associated with physiological changes with the endocrine system being part of the adaptive mechanism to reduce physical activity during sleep.

Renal system

Kidney filtration, plasma flow, and the excretion of sodium, chloride, potassium, and calcium all are reduced during sleep. These changes cause urine to be more concentrated during sleep.

There is also sleep-related increase in plasma aldosterone levels; an increase in prolactin secretion. There is increased parathyroid hormone release during sleep, which may affect calcium excretion. In general, the following are reduced during sleep: glomerular filtration rate, renal plasma flow, filtration fraction, and the excretion of sodium, chloride, potassium, and calcium. Smaller quantities of more concentrated urine are excreted during NREM sleep than during wakefulness; during REM sleep urine excretion is reduced and concentrated to a greater extent than during NREM sleep.

Alimentary activity

In a person with normal digestive function, gastric acid secretion is reduced during sleep. In those with an active ulcer, gastric acid secretion is actually increased and swallowing occurs less frequently.

Sleep and the athlete - psychomotor vigilance