Sheets are about feel and heat. Weave and fibre matter more than thread count, and no sheet has health benefits, though a breathable one helps hot sleepers.
The weave shapes how a sheet feels more than the fibre does. Percale is a plain, crisp, matte weave that breathes well; sateen is smoother and silkier but denser and warmer; flannel is brushed for winter; jersey is knitted and stretchy. Fibre matters too: cotton is the durable default, linen is airy and softens with washing, lyocell and viscose feel cool and smooth, and polyester microfibre is cheap and wrinkle-resistant but holds heat and moisture.
Most 'bamboo' and 'eucalyptus' sheets are regenerated cellulose (viscose/rayon or lyocell) chemically made from plant pulp, not bamboo fibre. US regulators have fined retailers for selling rayon as 'bamboo'.
No trials show that any sheet improves health. Lab studies show heat and humid heat under the covers disturb sleep, and one small study found people slept better at 17°C than 22°C while wool versus polyester bedding made no difference. So room temperature and overall layer weight matter more than which fibre your sheet is made from. 'Temperature-regulating' fabric claims rest on small, often industry-funded studies.
17 healthy young adults slept in the lab under different fabric and temperature conditions. Wool sleepwear shortened time to fall asleep versus cotton, there was no effect of wool versus polyester bedding, and people slept better at 17°C than at 22°C.
Review of thermal sleep studies: heat exposure increases wakefulness and reduces deep and REM sleep, humid heat makes it worse, and bedding and clothing largely determine how cold affects sleep.