In spite of the importance of sleep disorder in clinical practice, and the widespread use of hypnotics (Johnson and Clift, 1968), no simple technique for objectively recording sleep patterns in clinical situations is available. EEG recording during sleep (Oswald, 1969) or the use of motility beds (Hinton and Marley, 1959) are sophisticated techniques for use largely in laboratory conditions. Many clinical investigations of sleep disorders (Haider, 1968) or trials of hypnotics (Matthew et al., 1969), rely upon subjective assessments of the quality and duration of sleep by the patient, together with a night nurse's assessment largely based upon observed restlessness. Errors in the estimation of duration of sleep are particularly likely to occur in depressed patients where the illness itself produces an altered awareness of the passage of time (Orme, 1969), while nursing observations on sleep are unreliable (Kupfer et al., 1970). Similarly patients suffering from insomnia due to chronic pain or secondary to diffuse organic cerebral disease may misjudge the passage of time and their period of wakefulness during the hours of isolation during the night. Another group who particularly misjudge the passage of time in relation to sleep are those obsessional personalities, with overvalued ideas about the necessity for adequate sleep, who account for so many of the requests for hypnotics in general practice. Hypnotic drug-dependent patients overvalue their insomnia during their withdrawal from drugs, and again make inaccurate assessment of their sleep patterns.