The length of the free-running period (τ) affects how an animal re-entrains after phase shifts of the LD cycle. Those with shorter periods adapt faster to phase advances than those with longer periods, while those with longer periods adapt faster to phase delays than those with shorter periods. The free-running period of humans, measured in temporal isolation units and in forced desychrony protocols in which the day length is set beyond the range of entrainment, ranges from about 23.5 to 26 hours, depending on the individual and the experimental conditions (e.g., temporal isolation vs. forced desychrony). We studied 94 subjects free-running through an ultradian LD cycle, which was a forced desychrony with a day length of 4 h (2.5 h awake in dim light, ~ 35 lux, alternating with 1.5 h for sleep in darkness). Circadian phase assessments were conducted before (baseline) and after (final) three 24-h days of the ultradian LD cycle. During these assessments, saliva samples were collected every 30 min and subsequently analyzed for melatonin. The phase shift of the dim light melatonin onset (DLMO) from the baseline to the final phase assessment gave the free-running period. The mean ± SD period was 24.31 ± .23 h and ranged from 23.7 to 24.9 h. Black subjects had a significantly shorter free-running period than Whites (24.18 ± .23 h, N=20 vs. 24.37 ± .22 h, N=55). We had a greater proportion of women than men in our Black sample, so to check the τ difference we compared the Black women to the White women. Again, Black subjects had a significantly shorter free-running period (24.18 ± .23, N=17 vs. 24.41 ± .23, N=23). We did not find any sex differences in the free-running period. These findings give rise to several testable predictions: on average, Blacks should adapt quicker to eastward flights across time zones than Whites, whereas Whites should adjust quicker to westward flights than Blacks. Also, Blacks should have more difficulty adjusting to night shift work and day sleep, which requires a phase delay. On the other hand, Whites should be more likely to have trouble adapting to the early work and school schedules imposed by society. More research is needed to confirm these results and predictions.