A wearable device not detecting deep sleep does not necessarily mean it is absent unless inhibited by factors like sleep apnea, pain, alcohol, or environmental stimulation.

10 minutes of bright outdoor light within the first hour of waking anchors the circadian phase and improves sleep onset that night.

Morning sunlight exposure shifts the cortisol awakening response forward, improving daytime alertness.

Long-term morning sunlight reduces age-related macular degeneration risk.

Sleep regularity predicts all-cause mortality more strongly than sleep duration.

Tracking deep sleep on a wearable accurately reflects EEG-measured slow-wave sleep.

Caffeine has a half-life long enough that consumption after 2pm measurably degrades deep sleep in slow metabolizers.