Public health officials have acted dramatically to curb the spread of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and associated coronavirus disease 2019 (COVID-19). In some regions, actions to mitigate COVID-19 have been drastic, such as mandatory shelter-in-place regulations, while others have been more lenient. In either case, life has changed markedly for the much of our global population. Research conducted amidst crises of similar magnitude to COVID-19 has shown that sleep is disrupted during and after such events. For instance, research conducted after the 2003 SARS outbreak in China demonstrated an increase in insomnia symptoms associated with the onset of the outbreak. In the context of a natural disaster, researchers found that 40% of those who survived an earthquake in Japan reported sleep difficulties in the years following the disaster and 8% reported short sleep duration. Although previous literature would suggest sleep duration is likely to decline, there are several reasons sleep duration may increase across the globe during COVID-19. First, due to the highly contagious nature of COVID-19 and lack of a vaccine, social distancing and work-from-home recommendations and policies have been widely implemented to curb the spread of the virus. As much of the global population spent less time commuting, more time at home, and less time socializing, it could be that their sleep duration increased, as opposed to decreased as has been observed among previous crises. Sleep is a critical element of immune system function. Experimental studies have shown that inadequate sleep results in increased susceptibility to viral infection. Research has also shown a heightened ability to mount an immune response among those who obtain a healthy, sufficient duration of sleep. As the COVID-19 pandemic unfolds, surveillance of sleep duration may be important in order to identify poor sleep practices, and to develop evidence-based interventions and campaigns to enhance sleep in response to this crisis as necessary. To further our understanding of sleep during the COVID-19 pandemic, we analyzed smartphone sleep durations from individuals residing in London, Los Angeles, New York City, Seoul, and Stockholm before (January 2019 through April 2019) and after the onset of the COVID-19 pandemic (January 2020 to April 2020).