Biomedical Engineering Reference
In-Depth Information
(Hastings
2001
; Schwartz et al.
2001
; Zucker
2001
) .
The assessment of time or timing system in the
brain regulates annual rhythm such seasonal
cycles like sexual behaviour, energy metabolism,
food intake and hibernation. The photoperiod is
the strongest synchroniser of seasonal functions
in most species. For example, in Djungarian
hamsters (
Phodopus sungorus
), short days induce
reproductive inhibition, inactivity and weight
increase, contrary to animals kept in long days that
do not display these changes. In sheep, reversal
of the annual photoperiodic cycle causes the
breeding season to phase shift by 6 months;
reduction of its period to 6 months triggers
two periods of reproductive activity every year
(Malpaux et al.
1993,
2001
) .
Neurons in the SCN respond to light during
subjective night with an expression of the imme-
diate early gene c-
fos
. FOS reactivity in the SCN
following a light stimulus depends on the photo-
period history or exposure experience (Sumova
et al.
1995
; Vuillez et al.
1996
) . Clock gene
expression in the SCN displays photoperiodic
variations (Messager et al.
1999,
2000,
2001
;
Nuesslein-Hildesheim et al.
2000
) , and the daily
profile of arginine vasopressin (VP) messenger
ribonucleic acid (mRNA) differs in short and
long photoperiods (Jac et al.
2000
) . There is
also an evidence that the thalamic intergenicu-
late leaflet (IGL), a relay between the retina and
SCN, is involved in photoperiod integration
(Menet et al.
2001
) . Some photoperiodic spe-
cies, for example, ground squirrels (
Spermophilus
parryii
), exhibit endogenous circannual rhythms
under captivity and seasonally constant condi-
tions (photoperiod and temperature) for long
periods (Lee and Zucker
1991
; Gorman et al.
2001
; Zucker
2001
) . A morning oscillator (M)
adjusted by dawn and an evening oscillator (E)
adjusted by dusk in the mammalian circadian
system have been suggested (Pittendrigh and
Daan
1976
) . The phase relationship between
M and E reflects the day length to which the
subject has been exposed. The oscillators con-
trol the pineal gland, therefore being able to
define not only the time of day but also the time
of year (Schwartz et al.
2001
) . The hypothesis
of Daan et al. (
2001
) suggests that the circadian
pacemaker in the mammalian SCN consists of a
double complex of circadian genes (Per1/Cry1
and Per2/Cry2), which is able to maintain the
endogenous rhythm. These two types of oscilla-
tors are speculated to have slightly different
temporal dynamics and light responses. The
Per1/Cry1 (or M) oscillator is apparently accel-
erated by light and decelerated by dark, and the
Per2/Cry2 (or E) oscillator is decelerated by
light and accelerated by dark. Therefore,
changes in the activity of M and E oscillators
are likely to have an influence on the adaptation
of the endogenous behavioural programme asso-
ciated with day length.
5
Functions of Sleep
Sleep is essential to all mammals studied and varies
widely in species, age, pregnancy, health status,
etc. Sleep affects the metabolism, endocrine and
immune functions. Lack of sleep increases energy
requirements and impairs the immune defence.
Studies in cattle on duration of sleep during
different stages of lactation in dairy cows show
that total sleep time is about 4 h per day and is
significantly lower in cows 2 weeks after parturi-
tion compared to that during the dry period and
peak lactation ( Ternman
2011
) . Studies with rats
have demonstrated that while these animals nor-
mally live for 2-3 years, rats deprived of REM
sleep survive an average of only 5 months. Rats
deprived of all sleep survive only about 3 weeks
(Rechtschaffen
1998
) . In humans, extreme sleep
deprivation can cause an apparent state of para-
noia and hallucinations in otherwise healthy
individuals.
Many hypotheses have been advanced to
explain the role of this necessary and natural
behaviour (Rechtschaffen
1998
) . The following
examples highlight several of these theories:
5.1
Evolution of Sleep
Sleep is ubiquitous among mammals, birds and rep-
tiles, although sleep patterns, habits, postures and
places of sleep vary greatly in different subjects.
