“…One factor thought to contribute to interindividual differences in the timing of circadian rhythms is the preference for morning or evening activity patterns (Horne & Östberg, 1977). Although the timing of many physiological and behavioral rhythms are reported to differ between morning- and evening-type individuals (Bailey & Heitkemper, 1991; Foret, Touron, Benoit, & Bouard, 1985; Horne, Brass, & Pettitt, 1980; Horne & Östberg, 1976; Kerkhof, 1991; Kerkhof & van Dongen, 1996; Lack & Bailey, 1994; Lavie & Segal, 1989; Pátkai, 1971), many of those studies were confounded by the very differences in behavior used to define individuals as morning or evening types (Bailey & Heitkemper, 1991; Foret et al, 1985; Horne et al, 1980; Horne & Östberg, 1976; Kerkhof, 1991; Pátkai, 1971). For example, many studies did not collect data throughout the complete circadian cycle, limiting data collection to waking hours (Bailey & Heitkemper, 1991; Horne et al, 1980; Horne & Östberg, 1976; Pátkai, 1971), or they collected data from freely ambulatory individuals whose level or pattern of activity may have affected the physiological measures (Bailey & Heitkemper, 1991; Foret et al, 1985; Horne et al, 1980; Horne & Östberg, 1976; Kerkhof, 1991; Pátkai, 1971).…”