Sleep the sleep of the mindful, do not sleep the sleep of the heedless. For the mindful among the astute sleep only for rest, and do not purposely sleep through laziness.
The Holy Prophet said, 'My eyes sleep, but my heart does not.' When you go to sleep, have the intention to lighten your burden on the angels and disengage the self from its appetites, and to examine yourself by your sleep; be aware of the fact that you are incapable and weak. You have no power over any of your movements and stillnesses, except by the judgement and measure of Allah. Know that sleep is the brother of death. Use it as a guide to death, for there is no way to wake up in death or to return to correct action once you have missed it. Whoever sleeps through an obligatory, recommended or supererogatory prayer is at fault, and his sleep is the sleep of the heedless and the path of the losers; he is at fault.
Whoever sleeps after he has discharged his duties concerning obligatory and recommended prayers, and has carried out his responsibilities, sleeps a praiseworthy sleep. I do not know of anything that is safer than sleep for the people of our times who have attained these qualities: this is because people have ceased to guard their faith and to take care in observing their conduct. They have taken the left-hand path. When a sincere bondsman strives not to speak out of place, how can he avoid hearing that which would prevent him from not speaking unless there is a safeguard? Sleep is one such safeguard. As Allah said,
إِنَّ السَّمْعَ وَالْبَصَرَ وَالْفُؤَادَ كُلُّ أُولـئِكَ كَانَ عَنْهُ مَسْؤُولاً
In excess sleep there are many evils, even if it is done in the way we have mentioned. Too much sleep is brought about by excess drink, and excess drinking is brought about being excessively satiated. Both of these things weigh heavily on the self to keep it from obedience, and they harden the heart from reflection and humility.
Make your sleep your last affair in this world; remember Allah with your heart and your tongue. Let your obedience to Allah overpower your wrong doings and seek help from Him while you sleep, fasting until the morning prayer, since if you are awakened at night, Satan whispers to you, 'Sleep again, you still have a long night,' for he wants you to miss the time of intimate contemplation and exposure of your state before your Lord. Do not be distracted in seeking forgiveness at dawn, for at that time there is much yearning for those in devoted supplication.