Sleep research has demonstrated that sleep is characterized by periods of different levels of brain activity. People experience dreams during only one of these periods, known as REM (rapid eye movement) sleep. Test subjects who are chronically deprived of REM sleep become irritable during waking life. This shows that REM sleep relieves the stresses of waking life. Which one of the following, if true, most strengthens the argument? ['Other factors being equal, people who normally have shorter periods of REM sleep tend to experience more stress.', 'Test subjects who are chronically deprived of non-REM sleep also become irritable during waking life.', "During times of increased stress, one's REM sleep is disturbed in a way that prevents one from dreaming.", 'Only some people awakened during REM sleep can report the dreams they were having just before being awakened.']