# Implementing a Circadian Adaptation Schedule after Eastward Flight in Young Male Athletes
## 4 Discussion  
The aim of this study was to examine the effectiveness of a light exposure/avoidance schedule to facilitate circadian adaptation following eastward travel. A randomised, between groups design was utilised to examine the difference between two conditions (control, light exposure). The main finding was that there was no difference in urinary melatonin excretion between conditions. It took participants 7 days to adapt to the 8.5 h eastward time zone change, as demonstrated by the gradual reduction of subjective jet lag and the gradual increase in the excretion rate of aMT6s. However, the light exposure schedule did not facilitate faster adaptation in the intervention group. Several review articles have provided detailed guidelines regarding strategies to reduce or potentially eliminate jet lag using light or other phase shifting interventions (i.e., exogenous melatonin) when travelling [3; 16]. Indeed, experimental studies have attempted to implement bright light interventions in the field but have relied on subjective responses or have used salivary dim light melatonin onset (DLMO) as a measure of jet lag [17]. DLMO can provide an accurate biomarker for circadian phase, however, implementation of the protocol can be time-consuming and may be impractical for field studies [2]. While the circadian principles of bright light interventions are well understood, the practicality of implementing them successfully, and acquiring an objective measure of circadian phase in field settings is not. In isolation of the experimental protocol, this is the first study to utilise urinary melatonin as a daily bio marker of the circadian phase of athletes in a field setting. Demonstrated by the return to baseline of nighttime melatonin in both conditions, the measures utilised in this study can be used as a valid circadian biomarker for future field studies involving athletes. For many sports teams or athletes, it is impractical to travel with a staff member solely responsible for formulating and enforcing light exposure schedules to facilitate circadian adaptation. A potential solution is to formulate a pre-planned schedule around training and other activities that will optimise adaptation to the destination time zone. This study indicates that providing a hard copy adaptation schedule and daily electronic reminders may not enhance adaptation to an 8.5 h eastward time zone change in young cricket players. There are several potential explanations for why the light exposure schedule did not enhance circadian adaptation. It is possible that participants in the intervention group did not comply with the instructions and reminders to seek/avoid light. Assuming that the  
Figure 2: Subjective and objective variables as a function of day: self-reported bedtime (**A**); self-reported get up time (**B**); subjective sleep quality (**C**); urinary aMT6s relative to baseline (**D**); subjective jet lag (**E**); subjective sleepiness (**F**). Data are mean \(\pm\) SD and have been offset to aid interpretation. Filled circles represent the control condition. Open circles represent the intervention condition. Asterisks indicate significant differences to baseline across both conditions. Dotted line in figure D represents baseline aMT6s. KSS = Karolinska Sleepiness Scale; BL = baseline. instructions and reminders elicited the recommended behaviour in the intervention group, it is also possible that incidental light exposure resulted in a similar rate of adaptation in the control group. The intervention group was consistently closer to baseline measures of aMT6s compared to the control group between days 1-4 of the protocol (Figure 2). However, from days 5-8, the control group was consistently closer to baseline measures of aMT6s compared to the intervention group. It is plausible that the slightly slower rate of adaptation in the control group between days 1-4 resulted in a CBTmin that coincided with incidental sunlight exposure, thus increasing the rate of adaptation between days 5-8 (Figure 2).