Sleep

Testosterone is made largely during sleep, so short or fragmented nights drag it down directly. This is one of the cleaner natural levers — and one where a popular 'fix' (a CPAP machine) turns out not to be a testosterone treatment at all.

Made overnight

The rhythm: Testosterone climbs during sleep and peaks near waking — a rise that is sleep-dependent, not just clock-dependent, and needs roughly three-plus hours of consolidated sleep to build. Cut sleep and you cut the window in which most of the day's testosterone is produced.
The cost of short sleep: In a tightly-controlled study, one week of 5-hour nights lowered daytime testosterone ~10–15% in healthy young men — comparable to 10–15 years of ageing. Honest caveat: it was a small study that didn't fully replicate when sleep was cut from the other end of the night, so treat the figure as directionally true but timing-dependent. The practical target is 7–9 hours of consolidated sleep. Evidence ●●●○○

Sleep apnoea

Treat the cause, not the T: Obstructive sleep apnoea is reliably linked to lower testosterone. But a meta-analysis found CPAP alone does not meaningfully raise testosterone — obesity is the shared root of both the apnoea and the low T, and weight loss is the lever that fixes both. Don't expect a CPAP machine to be a testosterone treatment. Evidence ●●●●○

Protect the window

Seven to nine hours of consolidated sleep protects normal testosterone; chronically short or fragmented sleep suppresses it within about a week, and recovers when sleep is restored. It's free, fast-acting, and most men aren't doing it.

The Testosterone Guide