What is the single most underrated daily habit for health?

Getting outdoor light in the morning. It costs nothing, takes ten minutes, and affects more systems than almost anything else on the usual list — circadian timing, which improves sleep onset that night; alertness and mood during the day; and vitamin D, though the amount depends on skin tone, time and exposure. Indoor lighting is orders of magnitude dimmer than daylight even on an overcast day, so being near a window is not equivalent to being outside. Combining it with a short walk covers two habits at once and requires no equipment, membership or planning.