The recall of dreams is extremely unreliable, though it is a skill that can be trained. Dreams can usually be recalled if a person is awakened while dreaming. Women tend to have more frequent dream recall than men. Dreams that are difficult to recall may be characterized by relatively little affect, and factors such as salience, arousal, and interference play a role in dream recall. Often, a dream may be recalled upon viewing or hearing a random trigger or stimulus. A dream journal can be used to assist dream recall, for psychotherapy or entertainment purposes. For some people, vague images or sensations from the previous night's dreams are sometimes spontaneously experienced in falling asleep. However they are usually too slight and fleeting to allow dream recall. At least 95% of all dreams are not remembered. Certain brain chemicals necessary for converting short-term memories into long-term ones are suppressed during REM sleep. Unless a dream is particularly vivid and if one wakes during or immediately after it, the content of the dream will not be remembered.