 Organisms need a method of sleeping and waking in constant conditions.  Need to be able to predict timing of events like migration and hibernation in response to approaching environmental extremes.  Physiological readiness and synchronisation of individual organisms for mating.  Better food supply  Absence of predators/ competitors.  More favourable conditions.  May be an advantage to have some sort of biological clock.

 Daily Rhythms (24hrs)- diurnal, nocturnal, crepuscular. Synchronised by light/dark.  Tidal Rhythms (12.4 hrs)- rhythms of activity correlate with tides. Hiding/ feeding.  Lunar Rhythms (monthly)- spring tides and neap tides- may ensure best conditions for development.  Annual Rhythms (Yearly or seasonal)- migration, hibernation, aestivation, mating and rearing of young are all timed to exploit most suitable conditions and avoid seasonal harsh conditions.

 Regular, repeated patterns of behaviour require timing mechanisms.  Exogenous rhythms e.g. daily activity – daylight.  Endogenous rhythms  Many rhythms are controlled by a combination of both.  Free running- a continuing rhythm in constant conditions.- body temperature/ hormone production.  Most biological clocks need to be constantly reset by environmental cues.

 Zeitgebers (time-givers)- clock setting environmental cues.  Entrainment- regular resetting of the clock.  Phase shift- When the start of a period of the rhythm is changed so that it is earlier or later.  Actogram – A chart that shows an animals activity over a period of time.

 Schematic single and double plotted actograms and masking. (A) Wheel-running activity is plotted as an actogram with each horizontal line corresponding to one day. Black vertical bars plotted side-by-side represent the activity, or number of wheel revolutions. The height of each vertical bar indicates the accumulated number of wheel revolutions for a given interval (e.g. 5 min). The rho- and alpha-phase marked at the bottom of the actogram refer to rest and activity, respectively. The white and black bar at the top of the scheme depicts light (12 h) and darkness (12 h), respectively. (B) To better visualize behavioural rhythms, actograms are often double plotted by aligning two consecutive days horizontally (e.g. day 1 left and day 2 right). (C) Schematic actogram of a nocturnal animal kept in very short photoperiods (LD 6:6). Since the animal is only showing activity during the dark phases it seems to entrain to the prevailing LD cycle. (D) Parallel monitoring of body temperature reveals that this apparent entrainment is only masking. Although the readout parameter "activity" seemingly adapts to the new schedule, body temperature continues to cycle with its free-running period length implicating that the circadian clock of the animal is not entrained. LD, light-dark cycle.

 Circadian rhythms – light is key environmental cue.  Mammals- SCN (suprachiasmatic nuclei) which is found in hypothalamus of brain is the master circadian clock. N.gif

 SCN sends messages to pineal gland and the pineal glad produces the hormone MELATONIN (promotes sleep)  More melatonin is produced over 24 hours in winter then in summer.  Clock is entrained by light and can be upset by changed light/dark cycles.-  Jetlag.(tiredness, shortened attention span)  If SCN is destroyed, so is the animals circadian rhythm.  SCN plays a major role in the circadian rhythm of many hormones produced by the pituitary. neuro00/web3/Hollmelfig.jpg

 Shift workers/ immune system/ ability to learn/ birth and death times.