The instinct on a hot night is to wear as little as possible, and it usually backfires. Bare skin against a mattress traps sweat with nowhere to go, so you wake up damp. A thin layer of the right fabric moves moisture off the skin and lets it evaporate, which is what actually cools you. The right fabric is a short list. Cotton is breathable and cheap and it absorbs, which is good until it is saturated. Modal and Tencel wick faster than cotton and stay smoother against skin. Washable silk regulates temperature better than its reputation suggests, staying cool in heat and warm in cold, though it is the most demanding to wash. Linen is excellent in humidity and creases immediately, which nobody sleeping should care about. What to avoid is polyester satin, which looks the part and holds heat and sweat against you all night.

Sleepwear should be loose enough that air moves through it and not so loose that it wraps around you when you turn. Check the waistband: a wide covered elastic or a drawstring sits flat, a thin exposed elastic digs in after a few hours. Look for flat or covered seams, especially across the shoulders and down the side, since a raised seam under a shoulder blade is exactly the sort of small irritation that wakes people. Buttons on a pyjama top are a matter of taste but a front placket makes a set easier to get on and off when you are half asleep. Length matters for shorts: a slightly longer leg stops the ride-up that makes people give up on short sets entirely. Wash washable silk cold on a delicate cycle in a mesh bag, and hang it to dry, never tumble.

Buy the fabric, not the print. A cheap cotton set in a beautiful pattern will still be beautiful and still be soaked at two in the morning. Do not buy polyester satin sleepwear, whatever the price, and be honest about whether you will hand-wash silk before you commit to it. Sleeping in nothing is not the cooling hack it is sold as. If you take one thing away: choose one layer of a fabric that moves moisture, in a cut loose enough for air to pass through, and change it more often in summer than you think you need to.