To be asleep means your body and mind have entered a state of rest where you're not conscious of what's happening around you. When you fall asleep at night, your eyes close, your breathing slows, and you stop responding to most sounds and movements. You might dream, or hours might pass in what feels like minutes.

Sleep is essential for your body and brain. While you're asleep, your body repairs itself, your brain organizes memories and information from the day, and you recharge your energy. Scientists have discovered that kids need more sleep than adults because their bodies and brains are still growing.

The word can describe light sleep or deep sleep. Someone might be sound asleep, meaning they're sleeping so deeply that noise won't wake them. Or they might be sleeping so lightly that the smallest sound startles them awake.

You can also use asleep to describe a limb that has temporarily lost feeling because blood flow was restricted. When your foot falls asleep from sitting cross-legged too long, it feels tingly and numb. That pins-and-needles sensation happens as normal feeling returns.

People sometimes say someone is asleep at the wheel (or asleep at the switch) when they're not paying attention to something important they should be watching carefully.