CSCW studies of large-scale distributed practice in the sciences and elsewhere have taught us important things about space and place as props and barriers to distributed collective action, but they have had relatively less to say about time. This paper develops a heuristic of collaborative rhythms and points to the work of temporal alignment as a neglected but crucial element underpinning distributed collective practice in the sciences (and other spheres of collective activity). Specifically, we argue that joint scientific work is organized around four separate registers, or 'rhythms' - organizational, infrastructural, biographical, and phenomenal - and that efforts to align such rhythms constitute an important and under-recognized aspect of collaborative work. The ideas and examples are drawn from our own field studies around IT infrastructure and collaborative practice across a range of scientific fields.