TL;DR: The function/functions of sleep are still not known as mentioned in this paper, which implies that we do not understand why we have to sleep, and therefore, there are several biological theories, which are more or less hypothetical, trying to explain the nature of sleep.
Abstract: A good night's sleep has been considered restoring and refreshing. BELLOCK, BRESLOW (1972) showed that a serene 7-8 hours' nocturnal sleep is positively associated with health status and longevity. The function/functions of sleep are still not known. This implies that we do not understand why we have to sleep. There are several biological theories, which all are more or less hypothetical, trying to explain the nature of sleep