Sleep is the illusive energy of God (lit -the all-pervading deity) and naturally has its sway over all created beings. The kind of sleep which sets in when the sensation-carrying channels (Snáyu) of the body are choked by Sleshmá, which abounds in the quality of Tamas, is known as Támasi-nidrá. It is this sleep which produces unconsciousness at the time of dissolution or death. A man of Tármsika-temperament sleeps both in the day and night; one of the Rájasika-temperament sleeps either in the day or in the night; while sleep never visits the eyelids of a man of Sáttvika-temperament before midnight. Persons with enfeebled Kapham and aggravated Váyu, or suffering from bodily and mental troubles, get little sleep, and if at all, their sleep is of the Vaiká-rika or delirious type (ie. much disturbed) *. 33-34.
O Sus'ruta! the heart is said to be the primary seat of consciousness (Chetaná) in the animated beings. Sleep overcomes a man whenever the heart is enveloped in the illusive effects of Tamas. Sleep is the offspring of Tamas and it is the quality of Sattvam that brings on awakening. This is the fundamental law of Nature. The self-conscious individuality (Self), ensconced in the material frame of man which is composed of the five material elements, recollects through the agency of the mind (Manah), which abounds in the quality of Rajas, the renaissance of his by-gone existences, and wakens up in his psychic plane the pictures of good or evil deeds done by him therein. Dreams are but the embodiment of these recollections. The self or Jivatmá, though he sleeps not himself, is said to be sleeping, whenever the sense organs are overpowered by the illusive energy of Tamas. 35.
Day sleep is forbidden in all seasons of the year, except in summer and in the case of infants, old men, and persons enfeebled by sexual excesses, or in Kshata-kshina diseases and in case of habitual tipplers. A sleep in the day may be enjoyed after the fatigue of a long journey, riding, or physical labour, or on an empty stomach. It may be allowed as well to men suffering from the loss of fat, Kapham or blood, to those of scanty perspiration, or of dry or parched constitution; and also to those who have been suffering from indigestion and who may sleep for a Muhurta (48 minutes) in the day time. Those who have kept late hours in the night may sleep in the day for half the time they have watched in the night (and no more). Day sleep is the outcome of perverted nature and all the Doshas of the body are aggravated by a sleep in the day, bunging on many a troublesome complaints such as cout;h, asthma, catarrh heaviness of the body, aching or lassitude in the limbs, fever, loss of appetite etc. On the other hand, the keeping of late hours in the night develops symptoms (Upadrava) which are peculiar to the deranged Váyu and Pittam. 36.
* Such persons may get sleep only, when being tired and exhausted they cease to think of their affairs.