Sleepiness (also known as drowsiness) is medically recognized as a state of near-sleep, or a strong desire for sleep without feeling a decrease in one's physical energy levels.    This state is independent of a circadian rhythm  ; so, unlike sedation, this effect does not necessarily decrease physical energy levels but instead decreases wakefulness. It results in a propensity for tired, clouded, and sleep-prone behaviour. This can lead into a decreased motivation to perform tasks, as the increase in one's desire to sleep begins to outweigh other considerations. Prolonged exposure to this effect without appropriate rest can lead to cognitive fatigue and a range of other cognitive suppressions.
Sleepiness is most commonly induced under the influence of moderate dosages of a wide variety of compounds such as cannabinoids  , GABAergic depressants   , opioids  , antipsychotics   , some antihistamines  , and certain psychedelics such as LSA or 2C-C. However, it is worth noting that the few compounds which selectively induce this effect without a number of other accompanying effects are referred to as "hypnotics".
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