Since the discovery of rapid eye movement (REM) sleep (Aserinsky & Kleitman, 1953), it is well documented that every person spends on average about 20 percent of total sleep time every night in a REM stage. If the sleeper is awakened from REM sleep, dream recall rates are very high: 80 to 90 percent of the awakenings yield some kind of dream report (for comparison, Nielsen, 2000). Even after nonrapid eye movement (NREM) awakenings (Stage 2 or slow wave sleep), some mental content has been reported quite often (Foulkes, 1962; Wittmann, Palmy & Schredl, 2004). Some researchers advocate the hypothesis that the mind never sleeps, that is, dreaming of some kind is present during the entire sleep process. Despite the consistency of the physiological processes, the variability of dream recall in the home setting is considerably large. Some persons almost never recall any dream, whereas others can relate a detailed description of their nightly experiences almost every morning. In the present chapter, models and research findings that try to explain interindividual differences in dream recall frequency (DRF) between subjects, as well as intraindividual fluctuations within one subject, will be presented. The influencing factors can be divided into two groups: trait factors and state factors (Schredl & Montasser, 1996/1997). Trait factors are characteristics that are quite stable over time, such as personality dimensions, cognitive functioning (for example, visual memory), and creativity. Sociodemographic variables, such as age and gender, and the habitual sleep duration can also be classified as trait factors. On the other hand, state factors are short-term acting variables, such as nocturnal awakenings, presleep mood, or major life events. The distinction between state and trait factors, however, cannot always be made in an exclusive way. It is, for example, possible that the habitual sleep duration (long sleepers versus short sleepers) affects dream recall frequency, and the sleep duration changes from night to night of one person have an effect on dream recall, too. Therefore, some factors will be discussed in both sections. First, the models of dream recall that have been published in the literature will be outlined in the section Models of Dream Recall. For each approach, the applicability for explaining state and trait factors will be emphasized. In order to evaluate the empirical data adequately, measurement methods and methodological issues associated with measuring dream recall frequency (for example, reliability) will be discussed (Measuring Dream Recall Frequency), because these factors can affect the results of the studies presented in the section Factors Affecting DRF in various ways. The last section of Factors Affecting DRF comprises the attempt to sort out and bring together the most important influencing factors for which solid evidence was established by the research activities in this field. In the light of these findings, the theories will be evaluated and future directions for researchers will be outlined.
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