system
Given the following post, respond as if you're commenting on Reddit:
human
Why are deep vein thromboses such a risk while on bed rest and long trips but not while we sleep?
gpt
Virchow's triad describes the three categories contributing towards thrombosis- hypercoagulability, endothelial injury, and stasis. This is a simplified understanding, but sufficient for this purposes. The factor which changes between best rest/long trips compared to sleep is that of stasis. In bed rest, the period of time with less venous blood flow is prolonged. Also contributing to the DVTs within the bed rest cohort would be these are often less likely to be "healthy people" (they are receiving bed rest for a reason!), and other pathologies, medications, treatments, etc. may be increasing the risk of DVT beyond that of bed rest alone. In long trips, we are seated rather than lying whilst sleeping. The seated position is less conducive to blood flow compared to the lying position (ELI5: the veins kink off). Some have hypothesized that lesser air quality in commercial air travel contributes to the hypercoagulability of blood, and some experiments suggest this may be the case but the mechanism of action has not been elicited. 

Source: physician and researcher specializing in thrombosis.