Nervous system ↗Pressure meets circadian phase
Time awake generally raises homeostatic sleep pressure; the circadian system provides a varying drive for wake or sleep across biological time.
BRAIN & NERVES / 05
Sleep has structure.
Timing has biology.
Homeostatic sleep pressure and circadian timing interact with arousal networks, light, behavior, breathing, temperature, substances, symptoms, and environment. NREM and REM recur in changing proportions across the night.
N1 begins the transition
The lightest NREM stage bridges wake and sleep as eye movements, muscle activity, responsiveness, and characteristic electrical patterns change.
N2 stabilizes sleep
Sleep spindles and K complexes help define N2, which commonly occupies a substantial portion of adult sleep.
N3 raises slow-wave activity
Deep NREM sleep is marked by high-amplitude slow activity and is typically concentrated more heavily earlier in the sleep period.
REM changes the operating state
Rapid eye movements, vivid dreaming tendency, cortical activation, autonomic variability, and skeletal-muscle atonia characterize REM physiology.
Cycles change across the night
NREM and REM recur approximately cyclically, but cycles vary and later sleep generally contains more REM; one ideal percentage does not fit every night.
Light anchors the central clock
Retinal input to the suprachiasmatic nucleus shifts circadian timing according to biological time, intensity, spectrum, duration, and exposure history.
Body clocks coordinate imperfectly
SCN outputs, meals, activity, temperature, hormones, and local clocks synchronize tissues; abrupt shift work or travel can misalign them temporarily.