If you have congestive heart failure does swelling occur in legs only after prolonged sitting or does it happen even when lying down or during other activities?

In mild case, it will only happen in prolonged standing. However, in advanced cases, leg edema (leg swelling) persists throughout the day, somewhat improved in early morning when you just woke up then it gets worse as the day progresses. Activity without prolonged standing may help the edema. In general, most leg edema, regardless the cause, gets worse with prolonged standing due to gravity.

Generally occurs toward end of day and goes away overnight unless in severe stage. Swelling can start with in minutes of standing/getting out of bed. You can measure calf diameter every hour to see. Next sign maybe having go get up at night to pee.