Sleepiness can be described as an increased urge to sleep combined with a growing inability to stay awake without feeling a decrease in one's physical energy levels. Unlike sedation, this effect does not necessarily decrease physical energy levels but instead decreases wakefulness, resulting in a propensity for tired, clouded, and sleep-prone behavior. 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. 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.
Compounds within our psychoactive substance index which may cause this effect include: