A sheet that lets go at three in the morning is not a sign of a cheap sheet. It is a sign of a geometry problem, and once you see the geometry the fix is obvious.
The pocket is either deep enough or it is not
Mattress height plus any topper is the number that matters. A fourteen-inch pocket on a mattress that is fifteen inches with a pad has almost nothing left to grip underneath, so any tug lifts the corner over the edge.
Where the elastic runs
Cheaper sheets put elastic only at the four corners. Elastic all the way around gathers the whole perimeter under the mattress and distributes the pull, which is why an all-around sheet stays put on the same bed where a corner-elastic one fails. Listings such as https://sopito.us.com/ sell adjustable straps as a separate item precisely because sheets keep failing this way.
What straps actually do
Elastic straps clip to the sheet at two corners and run diagonally under the mattress, converting an upward pull at the corner into tension along the underside. Adjustable sets cover everything from a crib mattress up to a king.
The dryer is killing your elastic
High heat degrades the elastane in the casing. A sheet that fit for a year and then stopped is usually a sheet that has been through a hot dryer forty times. Wash warm, dry on low, and pull it out slightly damp.
Friction is a real variable
A quilted mattress top grips cotton. A smooth stretch knit on a foam mattress does not, so the same sheet slides on one bed and holds on another. Foam and hybrid mattresses are the usual culprits behind sudden corner failures.
What to check before buying anything
- Measure mattress height including toppers and protectors.
- Look at whether the elastic is all-around or only at the corners.
- Check that the sheet is the correct size, not the next one down.
- Stop putting sheets through a hot dryer cycle.
- Note whether the mattress surface is slick or quilted.
Restless sleepers and shared beds
Two people rolling in opposite directions apply forces a single elastic hem was never designed for. This is the situation where straps are worth the small cost, rather than a fourth set of sheets.
Adjustable beds and hospital beds
Any bed that folds works a sheet loose faster, because the mattress changes shape under the fabric. Straps are close to essential here, and they reduce the wrinkling that matters for anyone in bed for long periods.
One place not to improvise
In a crib, the answer is a correctly sized fitted sheet and nothing else. Safe sleep guidance is that the crib holds a firm mattress, a snug sheet, and the baby, without additional items added to the sleep surface.
The tuck that helps in the meantime
Hospital corners on the top sheet, and pulling the fitted sheet diagonally instead of straight down, both buy you nights while you sort out the real fix.