These are similar to the relaxation type exercises. In fact if you do them in bed before you sleep, it will relax you too, a double whammy.

It is contracting various muscle groups... glutes, thighs, calves, etc one group at a time one leg at a time. I also flex my ankles.

I start with the feet and move up and end with the glutes.

There is another one where you lie on your back flat and slowly flex the leg and drag the heel toward you. It is like walking in bed.
Don't put alot of weight on the heel, or too light a pressure, but go for a moderate amount. This is movement without impact stress and also moves blood around.

I do these if I wake up in the night and find it difficult to get back to sleep. Many relaxation tapes, have this muscle contract/hold/release, as a beginning to various meditation exercises.