1. Physically in one's bed without necessarily being asleep. I was in bed for a week when I had the flu. I didn't have the energy to do much of anything. I like to spend some extra time in bed on Saturday morning, just relaxing and reading a book.
2. Asleep in one's bed. Jenna's in bed already—she had a rough week. Your brother was looking for you last night, but you were already in bed, and he didn't want to wake you.
3. Having sex. I can't believe I caught them in bed together! How am I supposed to keep this a secret?
See also: bed