Sex and ancestry determine the free-running circadian period.

The endogenous, free-running circadian period (τ) determines the phase relationship that an organism assumes when entrained to the 24-h day. We found a shorter circadian period in African Americans compared to non-Hispanic European Americans (24.07 versus 24.33 h). We speculate that a short circadian period, closer to 24 h, was advantageous to humans living around the equator, but when humans migrated North out of Africa, where the photoperiod changes with seasons, natural selection favoured people with longer circadian periods. Recently, in evolutionary terms, immigrants came from Europe and Africa to America ('the New World'). The Europeans were descendents of people who had lived in Europe for thousands of years with changing photoperiods (and presumably longer periods), whereas Africans had ancestors who had always lived around the equator (with shorter periods). It may have been advantageous to have a longer circadian period while living in Europe early in the evolution of humans. In our modern world, however, it is better to have a shorter period, because it helps make our circadian rhythms earlier, which is adaptive in our early-bird-dominated society. European American women had a shorter circadian period than men (24.24 versus 24.41), but there was no sex difference in African Americans (24.07 for both men and women). We speculate that selection pressures in Europe made men develop a slightly longer period than women to help them track dawn which could be useful for hunters, but less important for women as gatherers.

Effect of slit lamp table design on neck position and the prevalence of neck pain in eye care professionals.

BACKGROUND: Ophthalmologists/Optometrists have a high incidence of neck pain. Little research has been conducted on specific equipment that predisposes these professionals to cervical discomfort. OBJECTIVE: Primary purpose: to determine if neck position is altered by slit lamp table design. Secondary purpose: to confirm the prevalence of neck pain in eye care professionals. METHODS: A survey of work-related pain was administered to 36 subjects (8 ophthalmologist, 2 optometrists, 26 technicians). The craniovertebral (CV) angle was measured in each subject in three separate positions (resting posture, best posture, slit lamp posture) between two different slit lamps/tables: slit lamp-deep and slit lamp-shallow. RESULTS: 79% of subjects reported neck pain in the last 6 months. The mean CV angle of all subjects at resting posture, best posture, and both slit lamp postures differed significantly. There was also a difference in CV angle between slit lamps. CONCLUSION: Neck pain is more prevalent in eye professionals than in the general population. The use of slit lamps promotes a forward head posture which decreases the CV angle, putting the user at risk for neck pain. By altering slit lamp table design, the CV angle of eye care professionals can be increased, reducing the risk for neck injury.

Circadian rhythms of European and African-Americans after a large delay of sleep as in jet lag and night work.

Jet travel and night shift work produce large changes in when people sleep, work and eat; a challenge that was not encountered during most of our evolution. Successful adaptation would require the internal, master, circadian clock to make large phase shifts to reduce the circadian misalignment between circadian rhythms and the times for sleep, work and meals. We compared African-Americans and non-Hispanic European-Americans in how much their circadian clocks shifted after a 9 hour phase delay of the light/dark, sleep/wake and meal schedule, which has similarities to flying west or sleeping in the daytime after night shifts. We also measured their free-running circadian periods using a forced desynchrony protocol with a 5-h day. European-Americans had longer free-running periods and larger phase delays than African-Americans. Correlations (among all subjects, just African-Americans and just European-Americans) showed that longer circadian periods were associated with larger phase delays. Larger phase delays, facilitated by longer circadian periods, reduce jet lag after westward travel and make it easier to work night shifts and sleep during the daytime after night work. On the other hand, a shorter circadian period, which makes one more of a morning-type person, is better for most people given our early-bird dominated society.

Circadian rhythm phase shifts and endogenous free-running circadian period differ between African-Americans and European-Americans.

Successful adaptation to modern civilization requires the internal circadian clock to make large phase shifts in response to circumstances (e.g., jet travel and shift work) that were not encountered during most of our evolution. We found that the magnitude and direction of the circadian clock's phase shift after the light/dark and sleep/wake/meal schedule was phase-advanced (made earlier) by 9 hours differed in European-Americans compared to African-Americans. European-Americans had larger phase shifts, but were more likely to phase-delay after the 9-hour advance (to phase shift in the wrong direction). The magnitude and direction of the phase shift was related to the free-running circadian period, and European-Americans had a longer circadian period than African-Americans. Circadian period was related to the percent Sub-Saharan African and European ancestry from DNA samples. We speculate that a short circadian period was advantageous during our evolution in Africa and lengthened with northern migrations out of Africa. The differences in circadian rhythms remaining today are relevant for understanding and treating the modern circadian-rhythm-based disorders which are due to a misalignment between the internal circadian rhythms and the times for sleep, work, school and meals.