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Deep sleep first, REM later
The two stages that matter
Sleep comes in light sleep, deep sleep, and REM, and the last two matter most. Deep sleep is when the physical body recovers. REM is when the brain recovers, reallocates the information collected during the day, and creates memory.
Because the body needs to recover first, the early night is mostly deep sleep with a minority of REM. Once that work is almost done, sleep flips to mostly REM with a minority of deep.
Temperature through the night
Body temperature also moves through the night with the circadian rhythm, and each stage wants a different temperature. Falling asleep calls for body temperature to drop as fast as possible, and deep sleep needs a colder setting than REM does. REM is when dreaming happens and the brain switches off many of the body's functions, so a body that is too cold won't let the brain into REM, because that would be dangerous. The best environment for REM is a thermally neutral one.
A bed that adjusts temperature hundreds of times a night can follow that curve, using the sleep stage, the room temperature, the phase of the night, heart rate, and heart rate variability as inputs. The aim is to help the body do what it would do if it were fully efficient.
Why does the body go limp in REM?
During REM, circuits in the brainstem block the signals that normally travel from the brain to the muscles. The result is a temporary loss of muscle tone called atonia. It keeps the body still while the brain produces vivid dreams. In REM sleep behavior disorder, that block fails and people physically act out their dreams.
How does body temperature move through the night?
Core body temperature follows a daily cycle set by the brain's master clock, a cluster of neurons in the hypothalamus. It begins to fall in the evening, reaches its lowest point in the early morning hours, and then climbs toward waking. Heat leaves the body through the skin of the hands and feet as blood vessels widen. Sleep tends to begin while that heat loss is under way.
Why does deep sleep come first?
Sleep pressure builds the longer an animal stays awake, and it is strongest at the start of the night. Deep sleep is defined by slow brain waves, and those waves are largest in the first sleep cycles. As the pressure drains away across the night, deep sleep shrinks while REM periods grow longer. A full sleep cycle lasts roughly 90 minutes.
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