As an awake organism falls asleep, the activity of its body slows down. Body temperature, heart rate, breathing rate, and energy use all decrease. Brain waves get slower and bigger. The excitatory neurotransmitter acetylcholine becomes less available in the brain. The organism will maneuver, as best it can, to create a thermally friendly environment—for example, by curling up into a ball if it's cold out. Reflexes remain fairly active. These characteristics apply to some degree during all non-REM sleep, which constitutes ~80% of all sleep in humans.