Sleep (niddā or seyyā) is a natural state characterized by reduced bodily movement, limited reaction to external stimuli and loss of consciousness. It has been estimated that the average person spends about a third of their life sleeping. The biological function of sleep seems to be to rejuvenate the body and the mind by giving it complete rest. However, sleeping beyond the required amount, insufficient or irregular sleep can seriously effect one's physical and psychological well-being, a fact that the Buddha was fully aware of. He told his lay disciples that sleeping after daybreak (ussūraseyyā) and being up all night (rattinuṭṭhāna dassinā) would have a negative impact on their lives (D.III,185). He also realized that because monks and nuns do not have to work and have ample free time that they can all too easily slip into the habit of sleeping rather than meditating or studying. Hence his frequent reminder to them that they should not be ‘fond of sleeping' (niddārāma, A.III,116; It.72; Sn.96). He said: ‘When one is lazy, gluttonous, snoozing and lolling on the bed like a great fat pig, he will be reborn again and again'(Dhp.325).
We read that the Buddha would sometimes take an afternoon nap or siesta (divaseyyā), although this was probably only after he had become quite old. An ascetic named Saccaka once asked him if he slept in the afternoon and he replied: 'I can recall that in the last month of the hot season after returning from my alms round and having eaten my meal that I would fold my robe into four, spread it out, lie down and go to sleep mindfully and fully aware.' Saccaka sniffed: 'Some would call that abiding in delusion' (M.I,250).
The Buddha observed that our behaviour while awake can sometimes have an effect during sleep. Being immoral or cruel for example, can cause insomnia or restless sleep. 'When the fool is on his couch or bed or lying on the ground the evil actions he has done in the past descend on him, settle on him, lie on him, just as the shadow of a mountain in the late afternoon descends, settles and lies on the earth. At that time the fool thinks, ssI have not done what is beautiful or skilful. I have not protected myself against the fearful...ss Then he is uneasy and troubled, he weeps and despairs' (condensed, M.III,164-5). Likewise, a person who has a loving disposition 'sleeps happily, wakes happily and has no bad dreams' (sukhaṃ supati, sukhaṃ paṭibhujjhati, na pāpakaṃ supinaṃ passati, AV.342).
Those practising meditation sometimes find that they become drowsy after meals (bhattasammada), start nodding (pacāla) and have to struggle to keep awake despite having had sufficient sleep. This is one of the recognized hindrances to meditation and the Buddha gave this practical advice for helping overcome it. 'When thoughts that encourage drowsiness arise you should not give attention to them. If having done this the drowsiness does not go you should ponder the Dhamma as you have learned it, you should examine it and think about it. If having done this the drowsiness does not go you should recite the Dhamma as you have learned it. If having done this the drowsiness does not go you should pull both ear lobes and rub your limbs with your hands. If having done this the drowsiness does not go you should get up and having washed your face look around and gaze upwards to the starry constellations. If having done so the drowsiness does not go you should imagine a light and focus on it whether it is day time or night. Thus open and clear the mind will become bright. If having done so the drowsiness does not go you should turn your senses inwards and walk up and down with full awareness. If having done all this the drowsiness still does not go you should mindfully lie down on your right side placing one foot on the other (and when rested) get straight up thinking: “I must not indulge in the pleasure of sleeping, reclining and torpor” ' (condensed, A.IV,86-7).