In a another study, Roenneberg's group looked at the timing of sleep and activity for eight weeks during the change to daylight savings time in 50 people, taking into account each person's natural clock preferences, or "chronotypes," which range from morning larks to night owls.
For both morning larks and night owls, their timing for sleep and peak activity easily adjusted when daylight savings time ended in the fall. However, it never adjusted to the return to daylight savings time in spring. This was especially true for night owls -- those who stay up late and sleep late.
"If we didn't change to daylight savings time, people would adjust to dawn during the summer and again to dawn in the autumn," Roenneberg. "But this natural adjustment is interrupted by daylight savings time," he said.
One expert believes daylight savings time is only one of the ways we try to fool our biological clock.
"It is not surprising that when you change our time to respond to something other than the sun and daylight that different chronotypes are going to have a difficult time," said Dr. Louis Ptacek, an investigator at the Howard Hughes Medical Institute and the John C. Coleman Distinguished Professorship in Neurodegenerative Diseases at the University of California, San Francisco.
"Before artificial lighting, humans tended to live much more by the sun cycle," Ptacek said. "Whereas, now, people stay up all night and turn the lights on, which affects our biological clock. There is no question that we have been changing our clocks long before daylight savings time came along."
So, it's not surprising that daylight savings time affects our internal clock, Ptacek said. However, it is no more unnatural than our use of artificial light, he noted.
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