We have a king size bed with a really thick and heavy mattress.
Before going to bed, I ensure that the fitted sheet is pulled tight around the mattress.
Every morning, without exception, the sneaky fitted sheet has crept out from underneath the mattress. The sheet has lost all of its tautness around the mattress and it feels messy and less comfortable than it would if it had stayed tight around the mattress.
Keep in mind that the mattress is incredibly heavy and lifting it out of the bed frame more than a few centimetres requires 2 people due to the awkwardness of the weight and size.
Any recommendations about simple ways to keep this sneaky fitted sheet in place?