Large cognitive fluctuations surrounding sleep in daily living

It is well recognized that cognitive output fluctuates surrounding sleep. Such fluctuations are rarely investigated in subjects’ daily living, partially due to the inaccessibility of sleep laboratory technology and cognitive testing. Here we leverage the continuous and long-term smartphone touchscreen interaction logs to study the patterns of cognitive output surrounding sleep and contrasted these to the patterns underlying physical activity captured from wrist-worn actigraphy. According to spectral density analysis, both cognitive and physical activity was dominated by diurnal (∼24 h) and infra-radian (∼7 days) rhythms. However, these rhythms differed in a domain-specific manner. The proxy measures of cognitive performance – tapping speed, unlocking speed, and app locating speed – contained lower-powered diurnal rhythm than physical activity. Still, the amount of smartphone usage showed the strongest diurnal rhythm – even when compared to the ambient luminescence levels experienced according to actigraphy. Interestingly, the cognitive rhythms were not in sync with physical activity, as cognitive measures peaked later in the day and on weekdays rather than weekends. The difference between cognitive and physical activity became vivid during bedtime and subjects routinely interacted with their smartphones during the actigraphy labelled sleep period. The cognitive measures in this period were worse in comparison to the hour before or after sleep. Therefore, smartphones can seamlessly capture the dynamic fluctuations in cognitive output including during spontaneous awakenings. We conclude that the rhythms underlying cognitive activity in the real world are distinct from physical activity and this discord may be a hallmark of modern human behaviour.