Chronotype-Dependent Changes in Sleep Habits Associated with Dim Light Melatonin Onset in the Antarctic Summer

Abstract

Dim light melatonin onset (DLMO) is the most reliable measure of human central circadian timing. Its modulation by light exposure and chronotype has been scarcely approached. We evaluated the impact of light changes on the interaction between melatonin, sleep, and chronotype in university students (n = 12) between the Antarctic summer (10 days) and the autumn equinox in Montevideo, Uruguay (10 days). Circadian preferences were tested by validated questionnaires. A Morningness-Eveningness Questionnaire average value (47 ± 8.01) was used to separate late and early participants. Daylight exposure (measured by actimetry) was significantly higher in Antarctica versus Montevideo in both sensitive time windows (the morning phase-advancing and the evening phase-delaying). Melatonin was measured in hourly saliva samples (18-24 h) collected in dim light conditions (<30 lx) during the last night of each study period. Early and late participants were exposed to similar amounts of light in both sites and time windows, but only early participants were significantly more exposed during the late evening in Antarctica. Late participants advanced their DLMO with no changes in sleep onset time in Antarctica, while early participants delayed their DLMO and sleep onset time. This different susceptibility to respond to light may be explained by a subtle difference in evening light exposure between chronotypes.

Keywords: Antarctica; DLMO; MEQ; circadian preferences.

Figure 1

Participants’ hourly light exposure in Montevideo (red) and Antarctica (blue) Light exposure (in lux) was measured by ambulatory wrist actimeters recorded in 1-min epochs. Hourly averages of light exposure were calculated for each participant for nine days in each condition, and hourly data of the whole sample are represented in each box plot. The dotted line represents the 1000 lux threshold that corresponds to outdoor daylight. Asterisks indicate significant hourly differences between Montevideo and Antarctica (paired Student’s t-test; * p < 0.05, ** p < 0.01, *** p < 0.001).

Figure 2

Absolute melatonin levels between from 18:00 to 24:00 in Montevideo (red) and Antarctica (blue). Melatonin levels were obtained at 1-h intervals. The expected nocturnal increase in melatonin levels in both locations was observed. Antarctic diurnal melatonin levels (18:00–21:00 h) were very low (rarely > 7 pg/mL) and significantly different from the respective Montevideo values from 18:00 to 20:00 h (Wilcoxon signed-rank test: ** p < 0.01). Black dots represent outliers.

Figure 3

Dim light melatonin onset (DLMO) as a marker of the individual circadian clock. (A) DLMO was not significantly different (Wilcoxon signed-rank test, p = 0.91) between Montevideo (red) and Antarctica (blue); (B,C) Examples from two participants showing melatonin levels from 18:00 to 24:00 in Montevideo and Antarctica. The individual DLMO was determined in Montevideo as the interpolated point in time at which the quadratic fit surpassed 2 SD above the basal melatonin level. The abscissa value at this point represented the individual Montevideo DLMO. The ordinate value at this point represented the individual threshold of melatonin levels (horizontal dotted line). The Antarctica DLMO was calculated as the time in which the melatonin levels reached the individual threshold (dotted line). Changes in DLMO between both conditions were different across individuals. (B) represents an example of an individual who delayed its DLMO in Antarctica; (C) represents an example of an individual who advanced its DLMO in Antarctica.

Figure 4

DLMO chronotype-dependence. Individual changes in DLMO between Antarctica and Montevideo (ΔDLMO = DLMOAntarctica – DLMOMontevideo) were correlated with participants’ circadian preferences (R² = 0.441, p = 0.019).

Figure 5

Chronotype-dependent changes in sleep habits between Antarctic and Montevideo. SL data from the last night before melatonin measurements in each condition. (A) In early chronotypes, sleep onset (SO) was significantly delayed in Antarctica; (B) In late chronotypes, SO did not change between Montevideo and Antarctica. Open circles represent outliers; (C) Individual changes in SO between Antarctica and Montevideo (ΔSO = SOAntarctica – SOMontevideo) were significantly different between early and late chronotypes. (** p < 0.01).