I’ve been reading the Vinaya recently, and I found something interesting from the Vinaya Commentary.
“Of those who are forgetful and not clearly comprehending as they fall asleep” means: of those who, abandoning mindfulness and clear comprehension, descend into sleep.
There, although for those falling asleep an indeterminate succession of life-continuum consciousness (bhavaṅga) occurs, and the succession of mindfulness and clear comprehension lapses, still at the time of lying down a mental determination should be made.
A monk who sleeps by day should sleep with zeal, thinking:
“I will sleep, and then get up before my hair has dried after bathing.”A monk who sleeps at night should sleep with zeal, thinking:
“Having slept this much of the night, when the moon or a star reaches such-and-such a position, I will get up.”And taking up one meditation subject — or another according to one’s inclination — among the ten meditation themes such as recollection of the Buddha, one should enter sleep.
For one who does so is said to “fall asleep mindful and clearly comprehending,” without abandoning mindfulness and clear comprehension.
Tesaṃ muṭṭhassatīnaṃ asampajānānaṃ niddaṃ okkamantānanti satisampajaññaṃ pahāya niddaṃ otarantānaṃ. Tattha kiñcāpi niddaṃ okkamantānaṃ abyākato bhavaṅgavāro pavattati, satisampajaññavāro gaḷati, tathāpi sayanakāle manasikāro kātabbo. Divā supantena yāva nhātassa bhikkhuno kesā na sukkhanti tāva supitvā vuṭṭhahissāmīti saussāhena supitabbaṃ. Rattiṃ supantena ettakaṃ nāma rattibhāgaṃ supitvā candena vā tārakāya vā idaṃ nāma ṭhānaṃ pattakāle vuṭṭhahissāmīti saussāhena supitabbaṃ. Buddhānussatiādīsu ca dasasu kammaṭṭhānesu ekaṃ aññaṃ vā cittaruciyaṃ kammaṭṭhānaṃ gahetvāva niddā okkamitabbā. Evaṃ karonto hi sato sampajāno satiñca sampajaññañca avijahitvāva niddaṃ okkamatīti vuccati.