During the year there changes in the photo and scotoperiod caused by seasonal variations. In order to keep its clock synchronised to these cues, an organisms must be able to reset its clock
In humans, resetting is applicable when considering human travel, especially the phenomenon of jet lag. When we cross time zones there appears to be a shift in environmental Zeitgebers, especially light, and to a lesser extent temperature. This causes a conflict between external rhythms and the person’s internal circadian clock. As the internal rhythms reset to local environmental Zeitgebers, they do so at different rates. This results in the fatigue, gastrointestinal complaints and short attention span that constitute the symptoms of jet lag. Resetting also has issues with shift work, as shift schedules often require the frequent resynchronisation of individuals to new time cues as a result of working shifts.
The reason an organism needs to reset its clock is that the changes in Zeitgebers may cause a phase shift in the clock of the organism. Therefore, to understand resetting we need to understand phase shifts, by looking at phase response curves (PRC).
Phase response curves are constructed by keeping an organism in constant conditions to establish a freerunning cycle. This is most often constant dark. Then a change in conditions is used to induce phase shifts. These are such things as light pulses (in constant dark), dark pulses (in constant light) or steps from light to dark.
The unit of time used in PRCs is circadian time. This does not necessarily correspond to “normal” 24-hour time. Circadian time is used to keep track of time in a freerunning cycle and is used to denote time of day in a biological clock.
In a freerunning cycle, the period length of a cycle is set as 24 circadian time units. Time 0 is considered to be the time of onset of activity in diurnal animals. For nocturnal animals, onset of activity is given as time 12. These times are then used to set subjective day, with times 0-12 being subjective day, and times 12-0 being subjective night.
Calculation of circadian time
Phase response curves
If organisms kept in constant conditions are exposed to a single perturbing signals (a light pulse introduced to organisms kept in constant dark will be used as an example) at different times over a 24-hour period, the phase shift caused is not always the same. At some times during the cycle, the phase shift will regress (delay) the cycle; at other times the phase shift will advance the cycle. At some points in the subjective day there are little or no shifts in the phase.
If the phase shifts obtained by a perturbing signal are plotted against circadian time, the result is a phase response curve.
Although the PRCs for each different species is different, examination of PRCs show that there are certain common features. Delay phase shifts occur when the light pulse is given early in the subjective night and around “dusk”. Advancing shifts are achieved when the light pulse happens late in the subjective night at around “dawn”. There are two places in a PRC where there appears to be little phase shift – towards the middle of the subjective day, and area called the dead zone, and around the middle of the subjective night called the singularity. The singularity occurs because it is at this point that there is a switch from delaying phase shifts to advancing phase shift. The reason for the dead zone is less clear. It may be that that there is a change in the sensitivity of the circadian pacemaking process. However it may also represent a decrease in the sensitivity of detecting the perturbing signal.
A generalised phase response curve
in response to light pulses
PRCs with dark pulses
Whilst most PRCs are constructed using light pulses on animals in constant dark, some are produced using pulses of darkness in otherwise constant light to induce phase shifts. If this method is used, the pattern seen with light pulses is reversed, with advances being seen early in the subjective night and delays being seen late in the subjective night. Such graphs are referred to as mirror images (despite the fact that the graphs are not exactly reversed).
Factors that affect PRCs
Phase response curves are not identical for all animals. Different species have different PRCs. Many of the differences arise due to lifestyle. Larger phase shifts can be obtained from smaller signals in nocturnal animals. For example nocturnal hamsters respond to a 15 minute light pulse, whilst diurnal house sparrows respond only to light pulses of 2-4 hours. This may be due to nocturnal animals having a greater sensitivity in detecting light, as their natural environments have only limited light (Binkley 1997).
As a rule the PRCs for nocturnal animals tend to have a longer delaying portion. This is because nocturnal animals tend to freerun with a period shorter than 24 hours and need to delay to correct to a 24-hour cycle. Diurnal animals tend to have a longer advancing portion and they freerun with periods longer than 24 hours and thus need to advance their clocks to reduce the cycle to 24 hours.
Using PRCs to explain phase shifts
PRCs can be used to explain many of the properties of circadian rhythms
Entrainment to an altering photoperiod over the year i.e. short days in winter and long days in summer have been explained by looking at PRCs resulting from the use of light and dark pulses. Entrainment to shortening photoperiod is attributed to the phase shifting sensitivity to dark as revealed by dark pulse PRCs; whilst the entrainment to long summer days is explained by the phase shifting sensitivity to light shown by light pulse PRCs.
The effects of light intensity may be explained by the differences in the size of advancing and delaying portions in nocturnal and diurnal animals. In diurnal animals there is a larger advancing portion. The effect of constant light should therefore be an advance in the phase, leading to a shorter period length. On the other hand nocturnal animals have a larger delaying portion, the effects of constant light therefore should be a delay in the phase with a resultant longer period.