We hypothesise that a significant proportion of doctors and nurses will be dehydrated at the end of their working day. Some, as in other occupations, may be dehydrated at the start of their working day. This is likely to be associated with impairment in cognitive performance at the end of the working day compared to the beginning and expected to be more pronounced after a night shift. There may be less noticeable difference amongst nursing staff given the protected break time. Dehydration and associated impairment in cognition is of important clinical value as it can impact patient care. participants will be involved in the study for two shifts (one day and one night), aiming at 15 medical and 15 surgical nurses as well as 15 medical (total 15 day and 15 night shifts from each group) and surgical doctors (total 15 day and 15 night shifts from each group). Those that do not work both day and night shifts will participate for only one shift and a new participant will be recruited until target number of shifts is achieved.