Carlos Solis
Two States of Sleep
Sleep isn’t a single state — it breaks into two very different physiological modes. One is REM sleep (rapid eye movement); the other is NREM sleep (non-REM). Each stage serves different functions, and researchers still don’t have a complete answer for why we sleep.
During NREM, brain activity slows, and a person who wakes in this phase can feel groggy or disoriented. In infants, roughly half of sleep is REM; by age two that share drops to about one-quarter, which is why REM seems especially important for early brain development.
How the Body Changes During Sleep
Pituitary gland
During NREM the pituitary releases growth hormone and secretes prolactin. Prolactin counteracts dopamine’s effects, which helps lower overall physiological arousal.
Heart
Heart rate slows by about 10 to 30 beats per minute, and blood pressure falls. Blood also redistributes: less flows to the brain and more goes to the muscles.
Mouth
Saliva production drops, so the need to swallow decreases. About 5% of adults grind their teeth at night, most often during the earlier sleep stages.
Limbs and muscles
Changes in circulation cause the hands and feet to swell slightly. During REM, muscles are nearly paralyzed to prevent acting out dreams, but between dreams people change position roughly 35 times a night.
Lungs and airway
Throat muscles relax, which narrows the airway during inhalation. That can cause snoring or brief pauses in breathing that last a few seconds, a condition called sleep apnea.
Bladder
Vasopressin levels rise, so the volume of urine that collects in the bladder decreases. The nighttime volume drops to roughly one-half to one-third of daytime levels.
Adapted from BBC Science Focus
