What the study found
Researchers at the University of Chicago took ten lean, healthy men with an average age of 24. Three nights of up to ten hours in bed, then eight nights of less than five.12
After a week of short nights, their daytime testosterone was 10–15% lower.12 Testosterone in men falls by roughly 1–2% a year with age — so one week of short sleep did what 10 to 15 years of ageing does.2
What it doesn't prove
Ten men, one week, a lab setting. It shows how fast short sleep can bite, not exactly what it does to you over years. It's still the easiest lever on the list to pull, and it costs nothing.
A sleep routine you'll keep
- Pick a wake time and keep it seven days a week. Bedtime follows.
- Set a hard stop you can check — laptop shut at 22:30.
- Track it for two weeks before you change anything else. The pattern shows up fast.
- Make it the first thing you report on each night, not the last.
When it isn't a habit problem
If you're in bed for eight hours and still wake up wrecked, or someone tells you that you snore and stop breathing, see your GP. Some sleep problems are medical, and no routine fixes them.