A study published in the Journal of Environmental Psychology, investigated daylight exposure and its relation with vitality in everyday settings on an hour-to-hour basis. The method consisted of experience sampling combined with continuous daylight measurement and a sleep diary during three consecutive days. Data collection was distributed over a full year.
Results revealed substantial inter- and intra-individual differences in hourly light exposure. The amount of light experienced was significantly related to vitality, indicating that persons who were exposed to more light experienced more vitality, over and above the variance explained by person characteristics, time of day, activity patterns and sleep duration during the previous night. This relationship was more pronounced in the morning, during the darker months of the year and when participants had experienced relatively low vitality during the previous hour.
Overall, the results provide support for acute effects of light exposure on feelings of vitality during daytime, even in everyday life. While light exposure can never replace sleep, it may produce a similar energy-boosting effect much like drinking a cup of coffee.