Circadian rhythms are rhythms that span 24 hours. However there are other types of rhythm to which many organisms are entrained. 

Ultradian rhythms

Ultradian rhythms are rhythms that have a period shorter than 24 hours. There are many different ultradian rhythms – eye blinks (24 eye blinks per minute in humans), respiration, heartbeat and sleep patterns.

Sleep patterns 

Although the sleep-wake cycle is a circadian rhythm, there are patterns within sleep as well. When we go to sleep, it is not one long snooze, but instead a set several sleep cycles that each last approximately 90 minutes. In the sleep cycles there are two main stages, non-REM sleep and REM sleep. 

In non-REM sleep there is a progression though 4 stages from stage 2 (an effective “baseline” state that occupies 50-65% of sleep time) to stages 3 and 4, often called slow wave sleep as an electroencephalogram (EEG) will show high voltage slow brain waves at this stage. After approximately 20 minutes of slow wave sleep there is a quick transition back to stage 2 sleep and from there into REM sleep. REM (rapid eye movement) sleep is characterised by rapid eye movements, depression of spinal reflexes and an increase in heart rate and respiration. During REM sleep, and EEG of a person’s brain activity produces a trace to similar to that when they are awake. However there is no ability to move skeletal muscles and the person is still in deep sleep. After 10-20 minutes of REM sleep there is a return to stage 2 sleep. This cycle is repeated every 70-90 minutes throughout the night with the slow wave stage getting shorter and the REM stage getting longer as the night progresses (Different Stages of Sleep site 2002). 

Infradian rhythms 

Infradian rhythms are rhythms that are longer than 24 hours. These can be rhythms that exceed 24 hours by a few hours; they may be cycles of a few days, a few weeks, a few months, a year or even of many years. 

Circalunal rhythms 

Circalunal rhythms are rhythms that are synchronised to the waxing and waning of the moon that forms a lunar month. A lunar month lasts 29.5 days. Many cycles are coordinated with the lunar month or with stages of the lunar cycle. Atlantic fireworms (Odontosyllis spp.) for example breed on a circalunal basis just before the fourth quarter of the moon throughout most of the year. Their breeding activity produces a luminescence that can be seen for miles. It has been noted since Greek and Roman times that the flesh of oysters and other shellfish increases and decreases with the moon (Cloudsley-Thompson 1980). 

Possibly the best known circalunal cycle is the human menstrual cycle. This averages approximately 29.5 days between the ages of 15 and 40 (Binkley 1997). Although it has been argued that the menstrual cycle is not a circalunal cycle because there is a variation in both period length and synchronisation between individual women, it is possible that the use of artificial light and the blocking out of the moon by curtains and blinds in modern life affects the timing of the cycle. On the other hand it may be that the cycle was once properly circalunal and that the length of the cycle is now just due to ancestral timing.   

Other lunar cycles include the pit building activities of antlions (Myrmeleontidae). Antlions dig bigger pits at full moon, than at new moon. This pit building activity occurs every 29.5 days even in constant conditions (Goodenough 1993).

Circatidal rhythms 

Circatidal rhythms are cycles synchronised by the tides. Whilst these do not have an effect on most organisms, the flood and ebb of the tides will affect littoral animals. As tides are affected by the phases of the moon (with unusually high tides being seen at the full and new moons, and unusually low tides being seen in the first and third quarters of the moon), circatidal cycles could be seen as being partly circalunal cycles. An example of a circatidal rhythm is the vertical migration of the marine diatom Hantzschia that at high tides descends into the sand, but at low tide rises to the surface (Binkley 1997). Whilst circatidal do not seem to be affected by inundation (wetting), it may be that mechanical agitation from the waves, water pressure or changes in temperature may act as Zeitgebers (Cloudsley-Thompson 1980). 

Longer rhythms

There are many cycles that are longer than the days or months just discussed. Of these the most common are circannual i.e. rhythms that last a year. Circannual rhythms include body weight, gonad size, nest building activity, antler shedding and replacement, moulting and reproduction. Palolo worms (Eunice viridis) for example breed during the last quarter of the moon in November when segments of the worm laden with sperm and eggs break off at dawn and rise to the surface of the water where the eggs and sperm are shed. Another species of Palolo worm (E. fucata) swarm in a similar manner in the last quarter of the moon in late June (Cloudsley-Thompson 1980). In many plants flowers are only produced at certain time of the year. 

The exact duration of a circannual cycle is influenced by many different factors. For some cycles the regulation is photoperiodic with an action occurring only when a critical photoperiod is reached. Starling moulting is photoperiodically controlled (Binkley 1997), as is Woodchuck body weight. Woodchucks transferred to Australia from Pennsylvania but kept in normal photoperiodic conditions had their weight cycles reset by the reversed annual cues gained by their transfer form the north to south hemisphere (Gwinner 1981). Hibernation cycles may be temperature controlled as transfer to a warm or cold temperature can phase shift the rhythm in ground squirrels.. 

CicidaThere are cycles even larger than circannual. Some bamboo species will flower once every seven years. Some species of cicada emerge every 13 or 17 years. Cicadas are members of the order Homoptera (which also includes aphids and scale insects). All cicadas have life cycles of longer than a year. With most cicada species there are adults present for most or all of the year. These are called non-periodic cicadas. However some species will mature into adults at the same time in any give location these are called periodic cicadas. During the 12 or 16 years between emergences adults of these species are almost absent, but in emergence years (every 13 or 17 years depending in species) they will emerge in huge numbers – a 13 year brood that emerged in 1989 (and will emerge again this year (2002)) spread from southern Indiana to the Mississippi River delta in the U.S. the emergence cycles are due to the life cycle of the cicadas. Once a cicada egg hatches, the 1st instar nymph falls to the ground from the egg nests in the trees and will burrow underground. The juveniles will then mature and grow over the next 13 or 17 years. When the juveniles are mature and ready to become adults they emerge over a small apace of time (normally a matter of weeks). The exact cue for this is not known but it is thought that soil temperature may play a role. Once emerged, the juveniles moult, mature, mate and die within a short number of weeks. The resulting eggs will hatch to restart the cycle in another 13 or 17 years time (Periodical Cicadias Page, 2002).

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