Core Sleep Medicine: Architecture, Neurobiology, and Homeostasis
What you must know
Sleep is staged by the AASM rules: N1, N2, N3, and R, each defined by EEG, EOG, and EMG criteria. Understand the two-process model: Process S (homeostatic sleep pressure, tracked by adenosine accumulation) and Process C (circadian timing from the suprachiasmatic nucleus). Know the ascending arousal system (orexin/hypocretin from lateral hypothalamus, cholinergic and monoaminergic nuclei) and the VLPO sleep-promoting nucleus.
Normal architecture across the night: N3 predominates in the first third, REM in the last third, with REM cycles lengthening toward morning. Normal adults have 4-6% REM at sleep onset? No-sleep-onset REM is abnormal in adults and suggests narcolepsy, severe sleep deprivation, or depression with hypersomnia.
Traps
- Confusing sleep latency (first epoch of any sleep) with REM latency (first REM epoch from sleep onset).
- Assuming N3 is evenly distributed; it is front-loaded.
- Misreading alpha intrusion in N3 as wake.
- Forgetting that spindles and K-complexes define N2 and that N2 continues even if spindles are absent once established, unless an arousal or stage change occurs.
Rule of thumb
Use "first third N3, last third REM" to sanity-check any hypnogram question. If a choice implies deep sleep dominates the morning, eliminate it.
Worked example
A patient's hypnogram shows REM at 8 minutes after sleep onset. Normal REM latency is ~90 minutes (range ~70-120). A short REM latency points toward narcolepsy or REM rebound; in a young patient with cataplexy, the MSLT will likely show 2+ SOREMPs, confirming narcolepsy type 1.