Sleep is one of the signs of Allah in this universe, as Allah says (interpretation of the meaning):
“And among His Signs is your sleep by night and by day, and your seeking of His Bounty. Verily, in that are indeed signs for a people who listen”
In principle, sleeping is to be at night, but sleeping during the day is also possible. Allah has created the day for work and going out seeking provision, and He has created the night for rest; so sleep takes place at night and work during the day. Allah says (interpretation of the meaning):
“And it is He Who makes the night a covering for you, and the sleep (as) a repose, and makes the day Nushoor (i.e. getting up and going about here and there for daily work, after one’s sleep at night or like resurrection after one’s death)”
“See they not that We have made the night for them to rest therein, and the day sight‑giving? Verily, in this are Ayaat (proofs, evidences, verses, lessons, signs, revelations, etc.) for the people who believe”
“And We have made your sleep as a thing for rest.
10. And We have made the night as a covering (through its darkness),
11. And We have made the day for livelihood”
Imam Ibn Katheer (may Allah have mercy on him) said:
i.e., among the signs is that which has been created for you of sleep by night and by day, in which you get rest and stillness, and exhaustion and tiredness are taken away; and wakefulness and striving for one’s livelihood have been ordained during the day.
Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allah have mercy on him) said:
“And We have made your sleep as a thing for rest” means: to put an end to tiredness, because sleep puts an end to that which comes before it of tiredness, and a person renews his energy thereby for what lies ahead. Hence you see that if a man becomes tired and sleeps, he is refreshed and his energy is renewed. This is a blessing and it is also one of the signs of Allah, as Allah says (interpretation of the meaning):
“And among His Signs is your sleep by night and by day, and your seeking of His Bounty”
“And We have made the day for livelihood” i.e., living whereby people seek their provision according to their status and circumstances. This is the blessing of Allah upon His slaves.
Tafseer Juz’a ‘Amma, p. 22, 23.
There is nothing in sharee’ah to suggest that a Muslim has to sleep at a certain time and wake up at a certain time, and Islam does not tell the Muslim to sleep a certain number of hours. Rather that depends on people’s ages and natures, and how much rest their bodies need, and it also depends on the nature of their work. One who works at night will sleep more during the day and one who works during the day will sleep more at night. Sleep during the winter is different than sleep during the summer and so on.
The natural average length of sleep is between 5 and 8 hours every day. If someone sleeps less than that because he is able to put up with that, or someone sleeps more than that because his body needs that, there is nothing wrong with that. What matters is that Islam obliges the Muslim to observe the times of prayer, at which time he should be awake so that he can perform the act of worship in the proper manner and with energy. If his body needs rest and sleep, he should not resist that.
Shaykh Muhammad ibn Ahmad al-Safaareeni (may Allah have mercy on him) said:
It is not right to resist sleep a great deal and stay up late often. Resisting and avoiding sleep will lead to other problems such as bad moods, muddled thinking, and exhaustion that prevents one from understanding and working properly, and it can lead to many fatal illnesses.
Creation is based on balance and whoever adheres to moderation has achieved all goodness. In al-Adaab al-Kubra, one of the wise men said: Drowsiness takes away reason, but sleep increases it.
Sleep is one of the blessings that Allah bestows upon His slaves, hence He reminds them of it in His Book.
It should be noted that sleeping more than the body needs will make one lazy in worship and will make one slow in thinking. Hence there are some sayings of the salaf which criticize sleeping too much.
Al-Fudayl ibn ‘Iyaad (may Allah have mercy on him) said: There are two qualities that harden the heart: sleeping too much and eating too much.
Ibn al-Qayyim (may Allah have mercy on him) said:
As for the five things that corrupt the heart, they are those that are referred to: mixing too much with people, wishful thinking, being attached to anything other than Allah, eating one’s fill, and sleeping. These five are the greatest corrupters of the heart.
He explained that which has to do with sleep and said:
The fifth corrupter is: sleeping too much, for it deadens the heart, makes the body heavy, wastes time, and generates a great deal of carelessness and laziness. Some of it is very makrooh and some is harmful and brings no benefit to the body.
The most beneficial of sleep is that which comes when the need for it is greatest. Sleeping at the beginning of the night is better than at the end, and sleeping in the middle of the day is better than sleeping at the beginning or end of the day. The closer sleep is to either the beginning or end of the day, the less beneficial it is and the more harmful, especially sleeping at ‘Asr time, and sleeping at the beginning of the day, except in the case of one who has stayed up late. Among makrooh kinds of sleep in their view is sleeping between Fajr prayer and sunrise, because that is a precious time. This time is of great benefit for those who are devoted to Allah; even if they had been worshipping all night, they would not allow themselves to stop at that time until the sun has risen, because it is the beginning of the day, when provision comes down and is divided, and blessings are bestowed. That is the beginning of the day and the whole day depends on what happens during that time, no one should sleep at that time unless he is compelled to.
To sum up: the best and most beneficial of sleep is sleep during the first half of the night, and the last sixth, which is equivalent to eight hours. This is the best sleep according to doctors. Anything more or less than that will have a proportionately detrimental effect on the body, in their view.
Another kind of sleep that is of no benefit is sleeping straight after sunset, until the twilight has disappeared. The Messenger of Allah (peace and blessings of Allah be upon him) disliked that, therefore it is makrooh according to sharee’ah and is naturally disliked. Just as sleeping too much leads to these problems, resisting and avoiding sleep will lead to other problems such as bad moods, muddled thinking, and exhaustion that prevents one from understanding and working properly, and it can lead to many fatal illnesses, whereby a person will not be able to think properly and will feel physically weak. Creation is based on balance and whoever adheres to moderation has achieved all goodness.
And Allah is the One Whose help we seek.
And Allah is the Source of strength.
(Source - Islam Q&A)