## 4 Discussion  
When people wake up earlier on weekdays, they miss out on the last portion of their sleep, but often believe they can make up for it during the following weekend. However, there is no way of minimizing the negative effects of insufficient weekday sleep by relying on a weekend \"catch up\". The simulations of weekday and weekend sleep timing suggested that, if people lose sleep by waking earlier during the week, they cannot make up for it on the weekends [11; 12; 13; 14; 15; 16]. An earlier waking time on weekdays leads to an earlier start and termination of light exposure of the internal circadian clock. This earlier exposure, in turn, leads to an advance in sleep timing and a larger loss in sleep on weekdays. This loss cannot be compensated for by the extension of weekend sleep duration [15]. Given that our work/study culture is biased towards the circadian clocks of M-types, the tradition of setting working and school start-times too early forces E-types to sacrifice a larger amount of sleep on weekdays to arrive at their place of work/study at the M-type-oriented start-times. As a consequence, E-types cannot simply catch up on sleep during the weekends, and this might be the major reason for complaints about \"the tyranny of the early risers/birds\" [17; 18]. Here, we applied a model of sleep-wake regulation to simulate sleep times of M- and E-types on weekdays and weekends to test the hypothesis that, since the literature indicates a >2 h difference between chronotypes in the positions of their endogenous circadian phases [5; 6; 7; 8; 9; 10], permitting a >2 h difference in the weekday waking times between M- and E-types would be necessary to equalize their irrecoverable loss in weekday sleep. The support for this hypothesis was provided by the simulations of sleep times reported by lecturers, and by the statistical analyses of sleep times reported by either lecturers or university students. We found that, to overthrow \"the tyranny of the early risers\", the >2 h difference between M- and E-types in waking times would ensure the equalization of their irrecoverable loss in sleep on weekdays. If, on weekdays, an \"average\" M-type university student or an \"average\" M-type lecturer wakes at 6 a.m., equality between the two distinct chronotypes in terms of weekday sleep losses would be achieved by enabling an \"average\" E-type university student or an \"average\" E-type lecturer to wake up no earlier than 8 a.m. The estimates of the difference in weekday sleep losses between E- and M-types appear to be comparable with the estimates reported in the literature for people before and after retirement [24] and for people living in adjacent counties on either side of a time-zone  
Figure 4: Simulated and self-reported wake intervals and bed- and risetimes. (**A**). Wake interval, (**B**) bedtime, and (**C**) risetime self-reported by survey participants (Table 3, middle and lower parts) in comparison with those obtained as output of simulations of the sequence of 10 sleep–wake cycles for two intervals, the interval of 5 weekdays, and the interval of two following weekends (Figures 1–3 and Tables 2 and 3, upper part). border [25]. The latter study also evaluated the negative consequences for health resulting from an additional reduction in sleep after a larger advance in weekday waking times reported by people living on the late sunset side of the border. The health index dropped by 0.3 standard deviations when people lived on this side of the border compared to the index of people living on the early sunset side [25]. Moreover, other reports indicated that these people are at an increased risk of cancer [26; 27]. The association of poor health with evening preference and late weekend sleep timing was previously noted [28; 29]. However, little is known about the contribution of a larger weekday sleep insufficiency in E-types compared to M-types. The association of being an E-type with poorer health can be a consequence of this type's tendency to develop unhealthy behavior [30; 31; 32; 33].