When we talk about sleep quality, we usually discuss the mattress, how dark the room is and our screen habits. The fabric touching your skin all night rarely gets a place in that conversation. Yet it is quite decisive.
As the body falls asleep, it needs to lower its core temperature slightly. That drop is part of moving into deep sleep. If the fabric against your skin makes it easier to release that heat and moisture, your body makes the transition comfortably. If it gets in the way, you wake up sweating in the middle of the night, throw off the duvet, then feel cold and cover up again. The next morning's feeling of "I slept but I'm not rested" often comes from here.
Breathable natural fibres, namely cotton, linen and wool, manage this balance of moisture and heat much better than synthetics. What matters here is thinking of the bed as a system. Pyjamas, duvet cover, duvet and mattress protector all work together. If you use a cotton duvet cover but polyester pyjamas and a plastic mattress protector, the weakest link in the chain sets the limit for the whole system. A small change, perhaps a single item, can change your nights more than you expect.