Sleepwear traps a thin private climate of air and moisture against your skin. What matters is how well the fibre absorbs sweat before it feels wet. Wool soaks up about 35% of its weight and keeps buffering; cotton around 24%, then sits cold and damp against you; plain polyester barely 1%, trapping a humid film.

In a warm room, adults aged 50 to 70 fell asleep in 12 minutes in wool, 27 in cotton, 22 in polyester ¹. In a cool room, wool sleepers nodded off in 10 minutes to cotton's 18 ². A review of nine trials found the same across labs: fibre changes how fast you fall asleep and how broken the night is ³. The effect is real but modest, the trials are small, and much of the wool work is industry-funded.

For men, there's a separate lever. Tight fabric pressed against the scrotum raises its temperature about 1°C — enough to slow sperm production, not enough to feel ⁴. Men who mostly wore boxers had 25% higher sperm concentration than briefs-wearers ⁵.

This is a decision, not a habit. Set your bedroom first, then dress to it. The sleep-medicine target is 17–19°C (62–66°F).

What actually changes. The first week, the common report is fewer 3 a.m. wake-ups in a damp shirt; the night stops splitting into two halves. Within a month the daytime version shows up as less mid-afternoon fade. For men who switch to boxers, sperm counts respond over three to six months, tracking the roughly 74-day production cycle ⁵. Sleepwear is the smallest of the three thermal levers: bedroom and bedding move more heat, and perfect pajamas can't save a hot room under a heavy synthetic duvet.