Before electricity, human activity followed the natural light–dark cycle, with sleep mostly at night. Electricity extended wakefulness into the evening and reshaped sleep patterns. Our studies on isolated Toba/Qom native communities in Argentina, offer a unique opportunity to explore the gradual shift in sleep patterns as they gain access to electricity and mobile devices. Based on over 10,000 nights of sleep recorded via actigraphy and sleep diaries, we monitored changes in sleep habits over more than a decade. This longitudinal data reveals a significant delay in sleep onset and reduced total sleep since electricity was introduced in rural communities (Casiraghi et al, under rev). In this follow-up study, we aimed to analyze the role of smartphone use in Toba/Qom sleep patterns. Our results show that smartphone use correlates with delayed sleep onset (13.4 min, p<.001) and shorter sleep duration (21.6 min, p<.001). This appears to have two causes: 1) a phase shift of the circadian clock, revealed by delayed melatonin onset in smartphone users; and 2) an increased procrastination of the time of sleep, with later sleep times on nights with reported smartphone use. These findings add to evidence that mobile devices can modulate sleep and circadian rhythms by both physiological and cultural ways, especially in communities newly exposed to technology. Our results underscore the importance of studying sleep in underrepresented populations undergoing rapid technological change.