Some people can quite accurately time the end of their night's sleep at will, without using an alarm clock, demonstrating that it is possible to voluntarily control a state of consciousness that is characterized by a loss of volition and attentional guidance. 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 adrenocorticotriopin(促肾上腺皮质激素) in the blood one hour before waking. The regulation of adrenocorticotropin release during nocturnal sleep is therefore not confined to daily rhythms; {{U}}(46) {{/U}} {{U}}(47) {{/U}} Normally, the release of adrenocorticotropin and cortisol(皮质醇)increases during late stages of sleeping, reaching a daily maximum at the time of spontaneous waking. Adrenocorticotropin and cortisol are also released from the pituitary-adrenal system in a major adaptive response to stress, and are secreted in anticipation of stressful events. We investigated whether the increase in the secretion of pituitary-adrenal hormones during the late stages of sleeping in part reflects anticipation of the 'stress' of the waking phase. {{U}}(48) {{/U}} We made recordings of electroencephalogram, electrooculogram and electromyogram throughout the night, and took blood samples every 15 minutes to determine plasma concentrations of adrenocorticortropin and cortisol. Lights were turned off at midnight, after subjects had been told they would be woken at either 6:00 ('short sleep', on one night) or 9:00('long sleep', on the other two nights). On one of the long-sleep nights they were woken at 9:00 as they expected, but on the other night they were instead woken at 6:00 ('surprise')under the pretence of a technical problem. {{U}}(49) {{/U}} We interviewed the volunteers at the end of the experiments, and found that all but one of the subjects had expected to be woken up at the specified time. The order of the three experimental nights was balanced across subjects, with five subjects starting with short sleep, five with long sleep, and five with the surprise condition. The increase in adrenocorticotropin release before the expected time of waking indicates that anticipation, which is generally considered to be a unique characteristic of the regulation of conscious action, pervades sleep {{U}}(50) {{/U}} The regulation of adrenocorticotropin release points to a mechanism that quickly adjusts endocrine activity to sharp changes in the duration.

E. The anticipatory adrenocorticotropin increase may also facilitate spontaneous waking F. About 10 million Americans consult doctors sleep problems each year