Sleep isn’t actually that easy to define because it is experienced by all animals (and perhaps even plants) in various different ways. It also involves many changes across the body and the brain, and involves distinct stages, which are all as different from each other as they are from waking consciousness. Contrary to what was once believed, we don’t just shut down and relax when we sleep. We might lie down and close our eyes, but our brains are anything but relaxed. In fact, in some ways, the brain is more active at night. Besides, sleep doesn’t always involve lying down, nor does it always involve having closed eyes!
Rather than trying to define sleep as a unitary concept, we can look at the four distinct stages of sleep that we humans go through across a typical night. Each stage has its own specific neurophysiology, and the kinds of dreams we have vary with the stage of sleep as well. See Stage 1 non-REM sleep, Stage 2 non-REM sleep, Stage 3 non-REM sleep, and REM sleep. See also hypnogram.
Sleep research is having a heyday at the moment. Thanks to the efforts of hundreds of sleep researchers across the world, we now know a great deal about why sleep is crucial for us. For one, it’s crucial for our bodies: sleep enables us to rest, recuperate, and heal. It’s also just as crucial for our brains and minds. Sleep enables us to strengthen our memories, process and regulate our emotions, think creatively, concentrate, and so on. Without sleep, everything goes wrong.