Blood pressure levels usually dip somewhat during sleep, but working the late shift seems to disrupt this natural diurnal cycle, according to US researchers. People who work evening and night shifts are less likely than day shift workers to show a 'dip'' in blood pressure levels while asleep, the researchers report in the September issue of the journal Hypertension. African-Americans are also less likely than members of other races to experience this blood pressure drop during sleep, according to Dr. Fumiyasu Yamasaki of The New York Hospital-Cornell Medical Center, and colleagues.