AbstractDue 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'st‐tests. On a weekly average, patients in DST slept less (difference between medians of 37 min.p < .01), mainly due to sleep on workdays (SD_w,p < .01), which also correlated with SoJL (r_sp = .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.