Light effects on circadian and homeostatic regulation: alertness increases independent of time awake.

Light induced improvements in alertness are more prominent during night-time than during the day, indicating circadian regulation or wake duration related dependence. Relative contributions of both factors can be quantified using a forced desynchrony (FD) designs. Here we investigate alerting effects of light in a novel 4x18 hours FD protocol (5h sleep, 13h wake) under dim (6 mlux) and bright light (1159 mlux) conditions. Hourly saliva samples (melatonin and cortisol assessment) and 2-hourly test-sessions were used to assess effects of bright light on subjective and objective alertness (electroencephalography and performance). Results reveal (1) stable free-running cortisol rhythms with uniform phase progression under both light conditions, indicating that FD designs can be conducted under high intensity lighting, (2) subjective alerting effects of light depend on elapsed time awake, while (3) light consistently improves objective alertness independent of time awake. Three dimensional graphs reflecting light induced alertness improvements depending on wake duration related variation and circadian clock phase suggest that performance is improved during daytime, while subjective alertness remains unchanged. This suggests that light during office hours might be beneficial for performance, even though this may not be perceived as such.