Your body runs on an internal clock that light sets each morning. When your natural sleep window and your schedule don't line up, the gap is social jet lag. Most adults carry one to three hours of it, five days a week, for decades ¹. Being a night owl is largely genetic: 351 spots in the genome nudge your timing, so it isn't a willpower failure ².

The regularity matters more than the hours. Every extra hour of social jet lag tracks with about a third higher odds of being overweight, even with total sleep held equal ³. In wrist-tracker data on hundreds of thousands of adults, the most irregular sleepers had 53% higher all-cause mortality than the most regular, and regularity beat duration on every endpoint ⁴. Seven hours every night beats nine on weekdays and five on weekends.

One anchor holds the whole thing up: a wake time you never move.

The clock is more state than trait, and it moves on a schedule.

• Weeks: afternoons stop crashing, and Monday stops feeling like a wall.
• A month or two: steadier patience, mood, and willpower on the days that used to be worst.
• Three weeks of this protocol pulled late types' clocks two hours earlier and dropped their depression scores ⁵.
• Years: the metabolic and mood markers move in the right direction.

Teenagers and shift workers are the exceptions. Around puberty the clock shifts about two hours later until roughly age 20 ⁷; aim for the earliest wake time school allows, then spend the effort on morning light. Shift workers are being asked to invert the clock rather than align it, and they carry a steeper risk profile ⁸.