Two kinds of nodding-off: absence or micro-dream - head erect, head hanging or resting

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From personal experience, backed-up by a bit of scientific theory, I think there are two ways in which a person might 'nod-off' and take a very short nap:

1. A momentary absence of awareness, as the mind briefly dips-into the shallower type of Deep sleep.

2. A fragmentary micro-dream, as the mind briefly dips into Rapid Eye Movement (REM) sleep.

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The 'absence' is what happens when somebody goes to sleep with their head held erect so that the neck muscles are tensed.

Probably, if the person was wired-up to an electroencephalogram, there would be some kind of regular slow wave activity such as occurs in the non-REM or Deep sleep and in petit mal epilepsy (also called absences).

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In Deep sleep the voluntary muscles (such as those holding up the head) become relaxed but not completely floppy - they retain muscle tone. So, when somebody who is sitting upright nods-off, their head slowly begins to slump until the person is roused from sleep when the head jerks upright again: giving the notorious 'nodding dog' appearance.

Therefore, unless someone is suffering from narcolepsy, neck muscle tension seems to block a descent from a state of being Awake directly into Dreaming sleep.

(The same applies to other bodily muscles of posture - their tension blocks descent into REM sleep; but it is rare for someone who is standing to nod-off.)

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However, if the neck muscles are relaxed, perhaps because the head is hanging forward, or if the head is resting on the hand, or a headrest - then a person can descend from being Awake and directly into Dreaming sleep.

This is possible because Dreaming (or REM) sleep is characterized by flaccid muscle paralysis - a near complete loss of muscle tone; and apparently a posture which maintains muscle tone makes is less likely to move directly into REM sleep, while a position which allows postural muscles to relax enables the mind to dream, instantly. 

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Subjectively, a brief Deep sleep nap is experienced as an absence - a cessation of awareness and memory; which may mean that sleep is not experienced at all (at least not consciously) - so, with the upright head/ nodding dog type of drowsiness, the person may not know that they have been asleep.

While by contrast a Dreaming sleep nap is experienced as Awareness being interrupted by snatches of dream. The person usually knows they have been asleep, because they recall (albeit perhaps only for a few seconds) the alien, bizarre, non-sequential quality of a dream.

But this is just a fraction of a second, because unless the body is supported, then the onset of flaccid paralysis will rapidly wake the sleeper as his head, body or limbs begin to slump, unresisting.

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