The circadian clock is the chronometer of life. Among other things, this ‘biological clock’ determines our waking, sleeping and mealtimes, and it influences our heart rate, blood pressure and body temperature. The clock is set by external stimuli, usually light, to follow a 24- hour rhythm. But what about organisms whose habitats are so isolated from the earth’s surface that they are never reached by a single ray of sunlight? In their research, Heidelberg zoologists address the question whether these animals have no sense of time at all, or whether they use other natural ‘time givers’ to regulate their activity and biological processes.