A recent, 1.5 years long record of acoustic Doppler current profiler (ADCP)-data from the Canary Basin (North-Atlantic ocean) likely reflects vertical zooplankton migration between 800 and 1400 m. This record clearly distinguishes 3 major periodicities of down- and upgoing motions to within a precision of ∼1/400: a daily, a seasonal and a monthly cycle. Largest daily excursions occur during full moon. The directly observed hourly mean vertical velocity amplitudes of ∣w∣ = 0.025 ± 0.01 m s⁻¹ are too slow for particles from the observational depths to reach the zone of moon- (and only very weak sun-) light penetration in half a day. It is shown that no physical (internal wave), geochemical or sinking food mechanism can trigger the daily and monthly cycles, which are coupled. It is speculated that an entrained biorhythm running precise internal biochemical clocks controls the vertical migration.