If sleep is prevented, people suffer and get worse at every kind of waking activity. From this it is clear that one important function of sleep is to maintain normal brain activity during awake time. Somehow, during sleep the brain gets restored to its normal functioning. Sleep is, so far as is known, universal amongst vertebrates. That also argues for its great importance. However, it is not known whether dreaming supports this repair function of sleep, or whether it is something which just happens.