The sheet slides off the mattress

Short answer: Short answer: A sheet slides when it is too big or lacks elastic all the way round. Measure the mattress height and choose a fitted sheet with elastic in the whole edge.

Height is the overlooked measurement

Most people choose a sheet by width and length and completely forget the height. A mattress that is twenty five centimetres deep needs a sheet with at least the same edge depth. If the edge is too shallow, the sheet only sits on the very edge of the mattress and pops off at the first turn during the night.

Elastic all the way round

A sheet with elastic only in the corners slides far more easily than one with elastic running the whole way round the edge. That is the one detail that matters most for whether a sheet stays put all night. See bamboo fitted sheets, where the elastic runs all the way round and holds the edge snug against the mattress.

When the mattress is smooth

A mattress with a smooth cover gives the sheet nothing to grip underneath. A thin underlay or a mattress protector with some texture under the sheet fixes this without changing anything else about the bed. Try this first if the sheet itself seems to fit the size just fine.

Adjustable beds need something else

If you have a bed that can be raised, the sheet needs to be able to follow the bend without pulling loose. A split sheet or an envelope sheet stays in place where an ordinary sheet pulls off. See bamboo split sheets and bamboo envelope sheets made for exactly that kind of bed.

Envelope sheets as an alternative

An envelope sheet holds onto the corners in a different way than a fitted sheet, and it suits narrower beds or beds with rounded corners well. Choose it if your fitted sheet keeps sliding even after you have checked both the height and the elastic all the way round the edge.

The mattress material plays a part

Memory foam and a traditional sprung mattress behave differently under a sheet. A softer surface can let the sheet sink in a little and hold better, while a firmer surface needs more elastic to keep the sheet in place through the whole night without working itself loose.

Washing and shrinkage

A brand new sheet can fit perfectly before the first wash and then become slightly too tight or too loose afterward, depending on the material. See bamboo sheets, where the quality is tested to keep its original size after repeated washes at the recommended temperature without noticeably changing the fit.

How to check the fit

Put the sheet on and gently pull diagonally at each corner. If it sits taut without giving way, it fits. If it gives a little at one corner, it is either too big or the edge is too shallow, and you should measure the mattress again before buying a new one.

Several sheets in rotation

If you only have one sheet, the elastic wears out faster than if you rotate between two or three. That gives each sheet more time to rest between washes and keep its stretch for longer, so you avoid ending up with a sliding sheet after only a few months of use.