1) During NREM sleep and tonic REM sleep, there is a relative increase in parasympathetic activity relative to sympathetic activity.
2) During phasic REM sleep, however, there are brief surges in both sympathetic and parasympathetic activity, resulting in a high degree of autonomic instability.
3) The autonomic nervous system reaches its most stable state during SWS in comparison to wakefulness.
4) Blood pressure, heart rate, and cardiac output decrease during NREM sleep, reaching their lowest average values and least variability in SWS.
5) Although these parameters remain reduced on average during REM sleep in comparison to waking, they attain their peak values during REM.
6) Respiratory rate and minute ventilation decrease during sleep, and upper airway resistance increases as a result of muscle relaxation, most significantly during REM sleep.
7) Brain and body temperature are down-regulated during NREM sleep, particularly SWS, as a result of a decreased hypothalamic temperature set point as well as active heat loss.
8) REM sleep, there is a decreased ability to regulate body temperature through sweating and shivering.
9) Growth hormone (GH) is released primarily during the early part of the night and its secretion is enhanced by SWS.
10) Sleep also stimulates prolactin secretion, although prolactin peaks after GH, usually during the middle portion of the night.
11) GH seems to enhance SWS, whereas prolactin may increase REM sleep.
12) In contrast, thyroid stimulating hormone (TSH) reaches its peak level in the evening just prior to sleep onset.
13) The hypothalamic-pituitary-adrenal axis (HPA axis) is usually at its most inactive state at nocturnal sleep onset.
14) Sleep onset inhibits cortisol release, whereas adrenocorticotrophic hormone (ACTH) and cortisol levels rise at the end of the sleep period.
15) Characteristics of REM sleep in men are the occurrence of penile erections, beginning in infancy and persisting into old age.