Why a new sheet can feel old
Small fibre ends can tangle where fabric rubs against the body, other laundry or itself. The weave, fibre and finishing affect how noticeable that becomes. A large thread-count figure on a label does not explain all of it.
Homeowners often focus on the number because it is easy to compare. The more useful questions are what the sheet is made from, how it is woven, how it feels in hand and how it is meant to be washed.
Check the care routine
Follow the temperature and drying instructions on the label. Avoid overloading the wash so the linen can move freely, and keep rough items with fasteners separate where practical.
If the sheet pills after only a few uses despite correct care, record when it was bought and ask the seller to review it before trying an aggressive depilling method.
Buying with fewer surprises
Feel the actual fabric and ask about composition, weave, fit and care. If sleep temperature is the concern, say that explicitly. A sheet that feels luxurious under showroom lighting still has to work on your mattress through an Indian summer.