Sleeping Disorder

If we define sleep as part of a clear cycle of activity and recovery so can we see that all creatures sleep. Even unicellular organisms exhibit a cycle of activity and recovery.
When we are active, we must use energy. This energy is stored in our bodies. When we consume this energy the body produces a waste. The brain stores all the time the information under our active time. It is primarily the brain recovers itself during sleep.
In the animal world there are large variations in the duration of different animal to sleep. Horses only need to sleep six hours while the bat is sleeping 18 hours each day. A dolphin requires a special solution to meet their need for sleep. The solution is that dolphin sleep with one brain awake. Predators can sleep almost anywhere and at any time while prey animals have to find solutions that protect them from predators when they are sleeping. One solution is staying in a flock, but there is always someone awake and can alarm.
The need for a safe sleeping place is important for us humans too. We have a bad smell, we have poor hearing and our sight is poor. In darkness, we see almost nothing.