In terms of heat stress management, there's an abundance of advice from a variety of sources. Government agencies including Health, Safe Work and Bureau of Meteorology all provide guidance to minimise the impact of hot weather. But what if such recommendations are based upon a lack of evidence, or worse, are contrary to the evidence. Consider the following messaging from Healthdirect.gov.au (reviewed October 2020) that states - "Avoid alcoholic, hot or sugary drinks (including tea and coffee) because these can make dehydration worse". There's a lot to discuss in that sentence so we'll focus on the caffeine from coffee and to a lesser extent tea.
Caffeine has been referred to as a diuretic since at least 1928, supported by research on high caffeine doses. Given that the vast majority do not consume such doses (an example is 6mg Caffeine per kg body mass; 100kg body mass = 6 cups of coffee - in one dose), the applicability of this research has been questioned and complemented with studies that better reflect typical consumption. Research assessing the diuretic effect of moderate doses of caffeine (an example is 4mg per kg body mass per day) suggests that coffee does not contribute to dehydration (Killer et al., 2014). Authors of a meta-analysis of 16 studies agree, stating "concerns regarding fluid loss and potential adverse effects on fluid balance associated with caffeine ingestion are unfounded" (Zhang et al., 2015). That's good news for those that enjoy a 'couple' of coffees at work. While many sources of information have been updated to reflect the evidence, frustratingly, more needs to be done to dispel the moderate dose caffeine - dehydration myth.
Reference
Killer SC, Blannin AK, Jeukendrup AE (2014). No evidence of dehydration with moderate daily coffee intake: a counterbalanced cross-over study in a free-living population. PLoS One 9;9(1): e84154.
Zhang Y, Coca A, Casa DJ, Antonio J, Green JM, Bishop PA. Caffeine and diuresis during rest and exercise: A meta-analysis. Journal of Science and Medicine in Sport 18(5): 569-74.
Disclosure - we are not coffee aficionados, our office does not even contain a coffee machine.