Most people are one or the other–that is, if they are back sleepers to begin with.
Tippers draw one knee up, poised and balanced, but without full consciousness to keep the muscles taut, the leg tips over to the side, usually crashing against the other sleeper in the bed. (Very rarely does the leg on the personless side of the tipper ever draw up to this position.) Until kicked or shoved over onto their side, tippers will repeat this motion a few times during the night. I am a tipper.
Sliders are worse. Sliders start out the same way, with one knee drawn up, but lacking traction, the leg slides slowly back down into prone position. This is not annoying. What is, is that sliders immediately react in their sleep to whisk that knee back up into what becomes starting position, to repeat the slow slide over again. When the knee is quickly drawn up, the other sleeper in the bed is immediately awakened–in time to witness the constant slide–kneejerk–slide–kneejerk repeated through the night until in exasperation, gets out of bed and heads for the computer. Often, at four-o-clock in the morning.
The Lost Children: A Charity Anthology