Short-form video platforms have become central to debates about digital overuse and sleep disruption, yet causal evidence on their behavioral impacts remains limited. We study the effects of adopting short-form video in the context of TikTok, which pioneered the format and accounts for a large share of global short-form video consumption. Using granular smartphone touchstream data from a large opt-in panel of U.S. Android users, we track app usage, screen time, and inferred sleep behavior before and after TikTok adoption. We employ a Difference-in-Differences (DiD) framework leveraging a high-frequency app usage dataset from nearly 7,000 TikTok adopters. Our findings reveal a significant increase in overall screen time---averaging 221 minutes per week for heavy TikTok users---following TikTok adoption, alongside notable declines in usage of other social media and gaming apps, suggesting a strong substitution effect. The screen time effects are highest during late night hours. We also observe shifts in sleep patterns, including delayed bedtimes and a moderate reduction in sleep duration, among high-intensity users. We observe no effects for low-intensity users, who engage minimally with TikTok post-adoption. Our findings show that short-form video adoption expands total digital attention rather than merely reallocating it, with important implications for platform design, regulation, and digital well-being. As a side result, our analysis provides clear evidence of substitutability between TikTok and traditional social media platforms, which has immediate implications for ongoing antitrust proceedings.