Humans come in different chronotypes: The phase of their sleep-wake cycle with respect to the phase of the external, sidereal cycle of night and day differs. Colloquially, the early chronotypes are addressed as "larks," the late ones as "owls." The human chronotype can be quantified in hours and minutes of local time by determining the median of the sleep phase. Demographically, early and late human chronotypes differ with respect to the stability of their rhythms and the prevalence of several widespread diseases and risk factors, such as depression, nicotine abuse, and others. Inbred mice are widely used in chronobiological research as model organisms, but up to now there was no way to chronotype them. We have developed a method to chronotype mice in hours and fractions of hours by measuring the median of activity (MoA) and have shown that different mouse strains have significantly different MoAs and, thus, chronotypes. We have further developed methods to estimate the stability of the behavioral rhythms and found that "late" mice have relatively instable rhythms. Our methods permit the use of inbred mice for investigations into the molecular and genetic background of the chronotype and the prevalence of risks and diseases that are associated with it.