According to a review of 61 studies of patients suffering from severely debilitating depression, a large majority of the patients reported that missing a night' s sleep immediately lifted their depression. Yet sleep-deprivation is not used to treat depression even though the conventional treatments, which use drugs and electric shocks, often have serious side effects. Which of the following, if true, best explains the fact that sleep-deprivation is not used as a treatment for depression? ['The dramatic shifts in mood connected with sleep and wakefulness have not been traced to particular changes in brain chemistry.', 'Keeping depressed patients awake is more difficult than keeping awake people who are not depressed.', 'Depression returns in full force as soon as the patient sleeps for even a few minutes.', "For a small percentage of depressed patients, missing a night's sleep induces a temporary sense of euphoria."]