Bed gravity is an irresistible¹ force that draws you back to bed, or toward any mattress², couch, or other soft horizontal surface. It is usually stronger when one or more persons are already sleeping on that furniture.

床重力是一股让你难以抵抗的力量，能将你拉回床、床垫、沙发或者其他一切软的可以平躺的物件上。如果已经有人躺倒在这些物件上，床重力的威力就会更加强大。

For example:

I have to fight bed gravity for at least 30 minutes every morning before struggling to get up.

我每天早晨从床上爬起来之前都至少要跟床重力斗争半个小时。