We will be using the sign for "sleep" for "time to go to sleep" and "sleepy". To perform this sign, you put your palms together with hands flat, then put them on the side of your face like putting your head on a pillow.
When teaching baby to sign, it is important to say the word while doing the sign. Make sure the baby is looking at you and paying attention. This helps baby to build language skills. Here is a video demonstration for "sleep".