Wakefulness is an increased ability to stay conscious without feeling sleepy combined with a decreased need to sleep.  It is contrasted with stimulation in that it does not directly increase one's energy levels above a normal baseline but instead produces feelings of a wakeful, well-rested, and alert state.   If one is sleepy before using this substance, the impulse to sleep will fade, keeping one’s eyes open will become easier, and the cognitive fog of exhaustion will be reduced.  However, sufficiently accumulated sleep deficiency can overpower or negate this effect in extreme cases. 
Wakefulness is most commonly induced under the influence of moderate dosages of a wide variety of compounds such as stimulants, nootropics, and 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 eugeroics or wakefulness-promoting agents. These include modafinil     and armodafinil  .
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