Contrary to popular belief, the human brain remains in a very active state during sleep, and being asleep is by no means equal to an absence of conscious experience. Previous research has shown that participants can produce a behavioural response to auditory stimuli during sleep without necessarily awakening. Such responses are elicited more readily during the subjectively lighter stages of sleep compared to the subjectively deeper stages. This is illustrated when we consider that auditory arousal thresholds (AAT's) normally progressively decline across the night, which is commensurate with the declining proportion of time spent in deep sleep as sleep progresses. Although the previously cited results describe the norm for many, it is problematic for the designers of alarm signals that AAT research has revealed that there are important individual differences that are likely to affect whether a sleeping person will respond to an auditory signal. These differences are as follows: Sleepy individuals (defined as sleep latency = 5 minutes), and alert individuals (defined as sleep latency = 10 minutes) who have been deprived of sleep, do not show the usual decline in AAT's across the night. Sleep deprived young adults will not reliably awaken to an alarm signal, regardless of sleep stage. Six to seventeen year old children will not reliably awaken to an alarm signal. Both the frequency of awakenings, and the intensity of a stimulus required to induce awakening, is related to age, with more frequent awakenings in response to lower stimulus intensity as age increases. Individual differences account for more variability in AAT's than sleep stage or age.
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