Although the rooster’s strident wake-up call just before sunrise feels as regular as an alarm clock, the evidence that the birds are somehow driven to herald the dawn has long been anecdotal. After all, a rooster is rarely shy about making his presence known regardless of the time of day.

Now, Japanese scientists have shed light on the phenomenon. To see whether roosters simply crow in response to external stimuli or according to a circadian rhythm — a pattern of behavior that occurs over a 24-hour period — the team put one group of birds into a room with continuously dim lights and tracked the birds’ crowing over a month.

Even in the absence of a daily cycle of light and dark, the authors report, the roosters continued to crow near dawn. However, over time their crows became more scattered, suggesting that the rhythm eventually faded as they adjusted to a new regime of perpetual twilight.