Background: Most research investigating the rate of circadian adaptation to transmeridian flight has focused on single, acute time zone transitions. Often however, aircrew experience compound time zone transitions, the physiological effects of which are not well understood. The primary aim of the current study was to investigate the circadian adaptation of a Royal Australian Air Force (RAAF) aircrew to several small time zone transitions using salivary melatonin onset as the marker of circadian phase.
Methods: Fifteen members of an RAAF aircrew collected saliva samples, sleep/wake records, and subjective alertness ratings during a 13-d surveillance patrol around the southwest Pacific Ocean.
Results: During the first 6 d of the surveillance patrol, the aircrew traveled 3.5 time zones east and melatonin onset advanced by 3.8 h. During the next 6 d the aircrew traveled 2 time zones west, but melatonin onset did not shift. Night-time sleep duration was shorter prior to workdays (6.4 h) than prior to rest days (8.4 h). Subjective alertness was not significantly affected by either the duration of night-time sleep prior to work, or the duration of flight.
Conclusions: The melatonin onset results indicate that participants' body clocks adapted well to several small time zone transitions when initially traveling eastward, but did not adapt to a similar pattern of time zone transitions when subsequently traveling westward. This finding is contrary to expectations based on studies of single acute time zone transitions which indicate that adaptation to westward flight is more rapid than adaptation to eastward flight.