Abstract Introduction Circadian clocks of morning-types (M-types) have earlier phases than evening-types (E-types) as measured by dim light melatonin onset (DLMO). We investigated whether older adolescent M- and E-types differ in self-selected daily light exposure when dark/sleep duration is fixed and consistent with their phase type. Methods The Smith Morningness Questionnaire was given to 74 older adolescents at screening. Those who scored in the top and bottom 10% were compared. There were 7 M-types (scored 43-49) and 5 E-Types (scored 19-25). These twelve adolescents (7 male; 18.5-20.9 years) followed an individualized 9-h sleep/dark schedule at home for one week with no other light exposure restrictions. Waking (15h) ambient photopic light was measured via a chest-level sensor (ActLumus, Condor Instruments). Then, salivary DLMO was measured in the lab. Each participant’s number of daily minutes ≥30, 100, 500, 1000, and 5000 lux were calculated. We identified the first (onset) and last (offset) times at which participants were exposed to bright light (≥500 lux) each day; clock times and hours relative to scheduled wake were examined. M-type wake times ranged from 07:00-09:00; E-type wake times ranged from 09:00-11:00. All daily values were averaged across the week. Results DLMO was earlier in M-types compared to E-types (21:06±00:48 vs. 23:33±00:36, p.001). Daily minutes ≥30 and ≥100 lux did not differ between groups. M-types received more minutes ≥500 (114±67.1 vs. 38.1±25.6, p=.040), ≥1000 (84.5±50.7 vs. 24.4±18.2, p=.031), and ≥5000 (39.6±24.2 vs. 8.0±7.5, p=.013) lux than E-types. Clock time of bright light onset was ~2h earlier in M-types compared to E-types (10:09±01:17 vs. 12:03±00:59, p=.020) but did not differ between groups when examined relative to wake. Clock time of bright light offset did not differ between groups. Bright light offset was ~2.5h later relative to wake in M-types compared to E-types (9.4±1.3h vs. 6.8±3.1h, p=.075), but this was only a trend. Conclusion Morning-type adolescents received 3-4 times more daily bright light (≥500, ≥1000, ≥5000 lux) than evening-types, which is likely driven by earlier waking behavior. Therefore, evening-types may be at a disadvantage for obtaining bright light. These differences may have downstream effects on circadian entrainment mechanisms. Support (if any) R01HL151512 (Crowley)
Poole et al. (Fri,) studied this question.