Circadian rhythm refers to the approximately 24-hour cycles in human and animal physiology and behavior, regulated by an internal biological clock. The master circadian clock is located in the hypothalamus, specifically the suprachiasmatic nucleus, which receives light input from the retina. These circadian rhythms evolved to protect DNA from UV radiation and help entrain organisms to the light-dark cycle. Core body temperature, melatonin secretion, and cortisol levels are classic markers used to measure circadian rhythms. Light exposure can advance or delay circadian rhythms depending on timing.

2. 
 Circadian rhythm is derived from the Latin words circa
dies meaning ‘approximately a day’.
 Term ‘Circadian’ was coined by Franz Halberg.
 It may be defined as the changes in human behaviour and
physiology that occur within a 24 hours period. The
human circadian system is regulated by endogenous
clock genes. There is a master clock found in the brain in
an anterior section of the hypothalamus nucleus known
as the suprachiasmatic nucleus and these are located in
the eye , brain , heart lung, gastrointestinal tract , liver
kidney and fibroblasts.
INTRODUCTION:

3. 
 ORIGIN:
 Photosensitive proteins and circadian rhythms are
believed to have originated in the earliest cells , with
the purpose of protecting the replicating DNA from
high ultraviolet radiation during daytime.

5. 
 The primary circadian “clock” in mammals is located in
the suprachiastmatic nucleus .SCN receives information
about illumination through the eyes. The retina of the
eye contains “classical” photoreceptors and the retina
contains specialized ganglion cells that are directly
photosensitive and project directly to the SCN where
they help in the entrainment of this master circadian
clock.
BIOLOGICAL CLOCK IN
MAMMALS

6. 
 These cells contains the photopigmentmelanopsin and
their signals follows a pathway called the
retinohypothalamictract , leading to the SCN . SCN takes
the information on the lengths of the day and the night
from the retina , interprets it and passes to pineal gland.
Pineal gland secretes the hormones melatonin. Secretion
of melatonin peaks at night and ebbs during the day and
its presence provides information about night – length.
Researcher at Harvard have shown that human subjects
can at least be entrained to 23.5 hour cycle and a 24.65
hours cycle.

7. 
The classic phase markers for measuring the timing of a mammal’s
circadian rhythm are:
 Melatonin secreted by the pineal gland.
 Core body temperature minimum.
 Plasma level of cortisol.
For temperature studies , subjects must remain awake but calm and
semi reclined in near darkness while their rectal temperature are taken
continuously. Though variation is great among normal chronotypes ,
the average human adult’s temperature reaches its minimum at about
05.00, about two hours before habitual wake time. Baehr et al. found
that , in young adults , the daily body minimum occurred at about
04.00 am for morning types but at about 06.00 pm at evening types.
This minimum occurred at approximately the middle of the eight
hours sleep period for morning types but closer to waking in evening
types.
BIOLOGICAL MARKERS AND
EFFECTS

8. 
 Melatonin is absent from the system or undetectably
low during daytime. Its onset in dim light , dim-light
melatonin onset at roughly 9 pm can be measured in
blood or the saliva. Its major metabolite can also be
measured in morning urine. But DLMO and the mid
point of the presence of the hormone in the blood or
saliva have been used as circadian markers.
Benloucif et.al found that melatonin phase markers
were more stable and more highly correlated with
the timing of sleep than the core temperature.

9. 
 Light resets the biological clock in accordance with
the phase response curve .depending on the timing,
light can advance or delay the circadian rhythms.
Both the PRC and the required illuminance vary
from species to species and lower light levels are
required to reset the clock in nocturnal rodents than
in humans.
LIGHT AND THE
BIOLOGICAL CLOCK

10. 
 Circadian rhythmicity is present in the sleeping and
the feeding patterns in human beings. There are also
clear patterns of core body temperature , brain wave
activity , hormone production , cell regeneration. In
addition ,photoperiodism , the physiological reaction
of an organisms to the length of day or night , is vital
to both plants and animals and the circadian system
plays a role in measurement and interpretation of
day length .
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IMPORTANCE IN HUMANS

11. 
 The rhythm is linked to the light – dark cycle.
Animals including humans , kept in total darkness
for extended periods eventually function with a free-
running rhythms. Their sleep cycle is pushed back
or forward each “day”, their endogenous period is
shorter or longer than 24 hours .
IMPACT OF LIGHT- DARK
CYCLE