Some people can quite accurately time the end of their night's sleep at will, without using an alarm clock1, demonstrating that it is possible to voluntarily control a state of consciousness that is characterized by a loss of volition and attentional guidance2. Here we show that the expectation that sleep will come to an end at a certain time induces a marked increase in the concentration of the hormone adrenocorticotropin in the blood one hour before waking. The regulation of adrenocorticotropin release during nocturnal sleep is therefore not confined to daily rhythms; it also reflects a preparatory process in anticipation of the end of sleep.