  This work analyzes time use surveys from 19 countries (17 European and 2
American) in the middle latitude range from 38{\deg} to 61{\deg} latitude
accounting for 45% of world population in that range. Time marks for primary
activities (sleeping, working and eating) are systematically contrasted against
light/dark conditions related to latitude.
  The analysis reveals that winter sunrise is a synchronizer for labor start
time below 54{\deg} where they occur within the winter civil twilight region.
Winter sunset is a source of synchronization for labor end times.
  Winter terminator also punctuate meal times in Europe with dinner times
occurring 3h after winter sunset time within a strip of 1h, which is 40%
narrower than variability of dinner local times.
  The sleep-wake cycle of laborers in a weekday is shown to be related to
winter sunrise whereas standard population's cycle appears to be irrespective
of latitude.
  The significance of the winter terminator depends on two competing factors
average daily labor time (some 7h30m) and winter daytime ---the shortest
photoperiod---. Winter terminator gains significance when shortest photoperiod
roughly matches to daily labor time plus a reasonable lunch break. That is
within a latitude range from 38{\deg} to 54{\deg}.
  The significance of winter terminator as a source of synchronization is also
related to contemporary year round time schedules: the shortest photoperiod
represents the worst case scenario the society faces.
  Average daily sleep times show little trend with the shortest photoperiod
slope 5min/h for a Pearson coefficient $r^2=0.242$. Average labor time may have
a weak coupling with the shortest photoperiod: slope 29min/h for $r^2=0.338$.
