The impact of daylight‐saving time (DST) on patients with delayed sleep‐wake phase disorder (DSWPD)

Abstract

Due to time zones, sun time and local time rarely match. The difference between local and sun time, which we designate by Solar Jet Lag (SoJL), depends on location within a time zone and can range from zero to several hours. Daylight saving time (DST) simply adds 1 h to SoJL, independently of the location. We hypothesised that the impact of DST is particularly problematic in patients with delayed sleep‐wake phase disorder (DSWPD), worsening their sleep debt. DSWPD is characterised by a chronic misalignment between the internal and social timing, reflected by an inability to fall asleep and wake‐up at conventional or socially acceptable times. We analysed the clinical records of 162 DSWPD patients from a sleep medicine centre in Lisbon, Portugal (GMTzone), and separated them into two groups: the ones diagnosed across DST or across Standard Time (ST). We included 82 patients (54.9% male; age: median [Q1, Q3] 34.5 [25.0, 45.3]; range 16–92; 54 in DST and 28 in ST) who had Dim Light Melatonin Onset (DLMO) measured as a marker for the circadian phase and sleep timing (onset, SO, mid‐point, MS and end, SE) self‐reported separately for work‐ and work‐free days. Differences between ST and DST were compared using Mann–Whitney or Student's t‐tests. On a weekly average, patients in DST slept less (difference between medians of 37 min. p < .01), mainly due to sleep on workdays (SDw, p < .01), which also correlated with SoJL (rsp = .38, p < .01). While the time from DLMO to SO was similar in those in ST or those in DST, the time from DLMO to SE was significantly shorter for those in DST. The average duration between DLMO and sleep end was close to 10.5 h in ST, the biological night length described in the literature. Our results favour perennial ST and suggest assigning time‐zones close to sun time to prevent social jetlag and sleep deprivation.

Spatiotemporal light exposure modeling for environmental circadian misalignment and solar jetlag

“…48,58 Solar jetlag, which is objectively measured using solar jetlag scores from our light exposure model, is defined as the mismatch between the social clock and the sun clock and is a source of chronic environmental circadian misalignment. Solar jetlag is a term coined by Reis et al 13 to refer to the extent to which people are living displaced from the sun clock. Although individuals located within a time zone adhere to a common social time, the amount of light from the sun to which a person is exposed will vary depending on location within a time zone.…”

“…This differential light exposure that drives solar jetlag is expected to exacerbate the consequences of social jetlag because of modern societal schedules. In particular, increasing solar jetlag due to westward location within a time zone would be compounded by the effects of work and school commitments occurring at early times (i.e., social jetlag), humans spending a large amount time inside buildings with relatively low light levels compared with daylight in the outdoors, and experiencing a lack of darkness during the night due to the pervasiveness and availability of artificial light, 13 all of which would culminate in less sleep duration. 13 In particular, the light exposure model captures environmental circadian misalignment (as the mismatch between the social clock and sun clock) due to geographic variation in the timing of light exposure as part of the ambient (outdoor) environment, which is impacted by location within a time zone, elevation, sunrise time, and sunset time.…”

“…In particular, increasing solar jetlag due to westward location within a time zone would be compounded by the effects of work and school commitments occurring at early times (i.e., social jetlag), humans spending a large amount time inside buildings with relatively low light levels compared with daylight in the outdoors, and experiencing a lack of darkness during the night due to the pervasiveness and availability of artificial light, 13 all of which would culminate in less sleep duration. 13 In particular, the light exposure model captures environmental circadian misalignment (as the mismatch between the social clock and sun clock) due to geographic variation in the timing of light exposure as part of the ambient (outdoor) environment, which is impacted by location within a time zone, elevation, sunrise time, and sunset time. Higher solar jetlag scores indicate residence in locations in the western region of a time zone that are higher in elevation (resulting in fewer obstructions to extend the field of view) and that are characterized by a later sunrise time and a later sunset time (and thus less light exposure in the morning and greater light exposure in the evening), which is hypothesized to promote evening wakefulness during times when sleep would otherwise occur.…”

“…This differential light exposure that drives solar jetlag is expected to exacerbate the consequences of social jetlag because of modern societal schedules. In particular, increasing solar jetlag due to westward location within a time zone would be compounded by the effects of work and school commitments occurring at early times (i.e., social jetlag), humans spending a large amount time inside buildings with relatively low light levels compared with daylight in the outdoors, and experiencing a lack of darkness during the night due to the pervasiveness and availability of artificial light, 13 all of which would culminate in less sleep duration. 13 …”

“… 4 Another source of circadian disruption is the timing of environmental light exposure due to geographic location within a time zone, which we hereafter refer to as environmental circadian misalignment or solar jetlag and is defined as the mismatch between the social clock and the sun clock. 12 , 13 In particular, individuals located within a time zone tend to adhere to a common social time (e.g., work day begins at 9 am local time). However, since the sun rises in the east and progresses to set in the west, individuals who are located in the western region of a time zone are subject to relatively less light exposure early in the morning and greater light exposure later in the day compared with individuals located in the eastern region of the same time zone.…”