Short Sleep Raises Evening Cortisol

Exhausted all day, electric at bedtime. That inversion isn't a personality flaw — it's what a short-sleep week does to the curve.

What the researchers actually did

Leproult, Copinschi, Buxton and Van Cauter restricted healthy young men to about four hours in bed for six consecutive nights, then sampled cortisol across the day and compared it with a fully rested condition.

What they found

• Evening cortisol was elevated after the sleep-restriction week.
• The rate of the daily decline slowed — the curve flattened out at the end of the day.
• Effects appeared quickly, within a single work week of short nights.
• Recovery sleep moved the numbers back in the right direction.

What it means for you

A run of short nights is self-reinforcing: less sleep raises evening cortisol, and higher evening cortisol makes sleep harder to get. Breaking that loop matters more than any single bedtime tweak.

What to do with it this week

• Protect the wake time and let bedtime drift earlier on its own.
• Treat two consecutive short nights as a signal to intervene, not a fact of life.
• Front-load hard tasks before 2 PM while your curve is still on your side.
• No new evening habits this week — just remove one.