Circadian rhythms play a crucial role in regulating physiology and behaviour. However, contemporary environments disrupt this system through structured schedules and electric lighting. The misalignment between biological and social time adversely affects sleep, performance, and health outcomes. This thesis integrates clinical, experimental, epidemiological, and digital methodologies to evaluate circadian misalignment's impact and design circadian-friendly environments.

In a randomised controlled trial (CircadianCare), modifications to structured hospital routines, defining personalised schedules based on patients' circadian preferences, improved sleep quality. To evaluate the effectiveness of a circadian hygiene intervention at a university, 3,185 students who withdrew early from the original 7,000 participants were assessed. Results showed variations in compliance by chronotype, with evening individuals facing more challenges (e.g., larger social jetlag, later sleep timing, poor sleep quality). A complementary investigation of lecture timetables and academic performance among 15,000 students revealed weak associations between class start times and exam results, with students starting early morning/early afternoon compared to mid-morning performing better.

Analyses of Google Trends revealed changes in the timing of digital searches following Daylight Saving Time (DST) transitions, suggesting that online search activity may be a useful surrogate marker of circadian disruption. A driving simulator experiment demonstrated that young male drivers experienced increased fatigue and impaired control following the spring DST transition, despite being unaware of their impairment. A review of DST and road safety highlighted discrepancies between epidemiological findings and the effects of sleep restriction and circadian misalignment (attributable to road safety policy, seasonal confounding, and daily illuminance variations). Finally, a longitudinal case study of an individual commuting across one time zone for 2.5 years showed earlier, fragmented sleep with no obvious resolution after the flight.

This thesis contributes to the field of chronobiology by providing insights into circadian misalignment in real-world scenarios and its implications for health, performance, and society.