It deadens the heart, makes the body heavy, wastes time, and gives birth to a lot of negligence and laziness. Some [types] of sleep are extremely disliked, and some are harmful to the body. The best form of sleep is what takes place when there exists a strong need for it. Sleep at the beginning of the night is more praiseworthy and beneficial than at the end of the night, and sleep in the middle of the day is better than at its beginning and end. The closer sleep is to the beginning and ending of the day the less the benefit and the more the harm. [This is] especially the case for sleeping in the after noon (‘asr) and sleeping at the beginning of the day, except for one who stayed awake all night.
And among the disliked aspects, according to the scholars, is sleep between Salaatul-Fajr and sunrise, for it is a time of special reward. For that time escape, among the saalikeen (travellers), is a great loss. Even if they traveled all night they would not allow sitting down from the journey at that time until sunrise, because it is the beginning of the day and its key. [It is] the time for provisions to be sent down [and distributed], the time for obtaining portions and the descent of blessings, and from it the day is established. The ruling regarding the whole day is [based on] the ruling regarding that portion. One should only sleep at that time if one is forced.
In general, the most balanced/moderate (a’dal) and beneficial form of sleep, is sleep during the first portion of the night and the last sixth of the night. According to medical practitioners, the length of the most balanced form of sleep should be eight hours. In their view, more [sleep] than that or less than that will cause deviation in one’s natural disposition according to it.
Among the forms of sleep that are not beneficial also, is sleep in the first part of the night shortly after sunset until the darkness of ‘Ishaa goes. The Messenger of Allaah (SallAllahu Alayhi Wa Sallam) used to dislike it. So it is disliked according to the Sharee’ah and naturally.
As excessive sleep causes these defects, putting aside sleep and abandoning it causes other major defects like bad temperament, dry personality and a warped disposition, as well as drying up of the humors which aid in understanding and work, and it leads to illnesses which will prevent one afflicted from benefiting either with his heart or body.
Existence [of the world] was established on the basis of justice (‘adl), so whoever adheres to moderation has grabbed his portion of the confluence of good.