Recent research has suggested that 7.5h sleep a night may not be sufficient to maintain adequate levels of alertness during the day. Two of the main arguments used in support of this theory are the ease with which many individuals fall asleep during the day and the ability of many to extend their nocturnal sleep length on demand. The first argument has been used to indicate an elevated level of daytime sleepiness, which may lead to decrements in performance throughout the waking day. The second argument uses the concept that all sleep is as a result of a physiological need, and so the ability to obtain additional sleep could indicate the repayment of a previous sleep debt. The first part of this thesis addresses the benefit of nocturnal sleep extension in terms of objective and subjective sleepiness in a group of young, healthy adults.