nodding

v. move the head up and down (usually indicating agreement or assent); fall asleep briefly, drop the head forward while dozing off; lose concentration momentarily

A nod of the head is a gesture used in many cultures that is most commonly, but not universally, used to indicate agreement, acceptance, or acknowledgment. "Nod" or "nodding" can also be used as a slang term to describe the physiological side effect related to consuming heroin or other opiates. Casual heroin users tend to nod as they drift between sleep and consciousness.
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Adjective
1. having branches or flower heads that bend downward; "nodding daffodils"; "the pendulous branches of a weeping willow"; "lilacs with drooping panicles of fragrant flowers"
(synonym) cernuous, drooping, pendulous
(similar) unerect
(classification) biology, biological science

Noun
1. a sign of assent or salutation or command
(hypernym) gesture, motion
2. the act of nodding the head
(hypernym) inclination, inclining
(hyponym) nutation
Verb
1. express or signify by nodding; "He nodded his approval"
(hypernym) gesticulate, gesture, motion
2. lower and raise the head, as to indicate assent or agreement or confirmation; "The teacher nodded when the student gave the right answer"
(hypernym) communicate, intercommunicate
3. let the head fall forward through drowsiness; "The old man was nodding in his chair"
4. sway gently back and forth, as is in a nodding motion; "the flowers were nodding in the breeze"
(hypernym) move
5. be almost asleep; "The old man sat nodding by the fireplace"
(hypernym) drowse

(p. pr. & vb. n.)
of Nod
  

(a.)
Curved so that the apex hangs down; having the top bent downward.
  

Synonyms and related words:
absent, absentminded, absorbed, abstracted, anesthetized, asleep, bagging, baggy, ballooning, bemused, castle-building, casual, cataleptic, comatose, daydreaming, daydreamy, distant, doped, dozy, dreaming, dreamy, drooping, droopy, drowsing, drowsy, drugged, drugged with sleep, ecstatic, elsewhere, engrossed, faraway, floppy, goofing off, half asleep, half-awake, heavy, heavy with sleep, heavy-eyed, in a reverie, in a stupor, in the clouds, incautious, incomplete, languid, lethargic, limp, loose, lop, lop-eared, loppy, lost, lost in thought, meditative, mooning, moonraking, museful, musing, napping, narcoleptic, narcose, narcotized, narcous, oblivious, off-guard, oscitant, out of it, pensive, pipe-dreaming, preoccupied, rapt, sagging, sagging in folds, saggy, sedated, sleep-drowned, sleep-drunk, sleep-filled, sleep-swollen, sleepful, sleeping, sleepy, slight, slumberous, slumbery, snoozy, somewhere else, somnolent, soporific, stargazing, stretchy, stuporose, stuporous, superficial, swag, taken up, transported, unalert, uncautious, unconscious, unguarded, unprepared, unready, unvigilant, unwary, unwatchful, woolgathering, wrapped in thought, yawning, yawny