Sleep Science

Sleep Stages & The 90-Minute Cycle

Deep sleep in the first half, REM in the second. Cutting sleep short cuts REM most.

Sleep ScienceFoundational sleep science

Adenosine · Sleep Pressure / Process S

The molecule that tracks how long you have been awake, and the one caffeine blocks to hide the bill.

Sleep NeuroscienceIV adenosine approved (Adenocard, 1989) for PSVT, not for sleep; no sleep indication exists

Body Temperature Rhythm as a Zeitgeber

You fall asleep as your core cools. Warm your feet, cool your core, sleep faster.

ZeitgeberFoundational biology

Sleep Trackers & Wearables · Measurement

These devices measure movement, heart rate, HRV and temperature. They infer sleep stages.

MeasurementGeneral sleep tracking is wellness, un-cleared. Specific features cleared: Apple Watch ECG/AFib (510k) and sleep apnea notification (cleared Sept 2024, US); Samsung sleep apnea (De Novo, 2024, US). Sleep-stage output is not FDA cleared.