The whole community keeps time. About 60% of gut microbes rise and fall on a ~24-hour clock, and roughly a quarter of their functional pathways oscillate daily. This programs the host’s OWN circadian genes, so the traffic runs both ways. Jet lag, shift work and late-night eating flatten the rhythm, which is linked to obesity and type 2 diabetes. This is the real, well-supported ‘cycle’ — a daily one.
This one acts on the whole community rather than a single taxon — see the description above.