A wool duvet is not automatically too hot for summer. A 2024 systematic review published through the National Institutes of Health compared bedding fibers and found wool outperformed both cotton and polyester on sleep onset and temperature regulation, in a room at 17 degrees Celsius and again at 30. Wool came out ahead in the cool room and in the warm one.
That result surprises people because they read warmth off weight. A heavier duvet feels like a warmer duvet, so a wool duvet sounds like a winter-only purchase. On the scale it often goes the other way. In the example this video works through, the wool duvet comes in roughly a pound lighter than the cotton duvet at each size, and it still carries a sleeper through winter. Weight is half the story, and the fiber does the other half.
The mechanism is moisture buffering. Wool takes up to 30 percent of its own weight in moisture as vapor, and when you heat up in the night, that vapor moves off you into the fiber and the heat leaves with it. When the room cools, the crimp in the fibers holds warm air in place around you. The duvet responds to the sleeper instead of holding one fixed setting from bedtime to morning, which is why wool can win at 17 degrees and at 30.
A synthetic fill works differently. It holds its warmth at one level, so a duvet that is right in January can be too much in July, and the usual fix is to buy a second, lighter one for the warm months.
The practical question is how far your bedroom swings. Sleep Foundation recommends a sleep temperature between 65 and 68 degrees Fahrenheit. Put a thermometer on the nightstand and write down the low and high every night for a week. If the room holds within a few degrees of that band on most nights, one duvet can cover the year, and wool's two-direction behavior is well suited to it. It also suits a couple where one person runs warm and the other runs cold, since a single fixed setting cannot satisfy both.
There are limits. If your coldest and warmest nights sit more than eight to ten degrees apart, or the room swings from 62 in January to 78 in July, a second duvet earns its place: a lighter cotton one for the warm months and wool for the cold ones. A sleeper who overheats whatever the thermostat reads may prefer cotton's cool-to-the-touch feel year round. And wool is not vegan, which settles the question for some households before temperature comes into it.
Originally published on [Circadian](https://circadianrest.com/blogs/pillow-talk/all-season-duvet-two-duvets-decide).
