tract connecting these ganglion cells with the SCN is called the reti-
nal-hypothalamic pathway, which communicates information about
levels of light to the SCN in order to synchronize the endogenous cir-
cadian rhythm with environmental time.
When we travel rapidly from one part of the world to another,
crossing multiple time zones, our internal biological clock continues
to operate on whatever time period it has recently synchronized with,
even though the external environmental time has shifted. This is jet
lag—a lack of synchrony between our internal clock and the new time
zone. If the internal clock is telling the body that it is time to sleep and
the local environment is dictating that we be awake and functioning,
the resulting jet lag can mean fatigue, headache, grogginess, fuzzy
memory, and other symptoms of impaired performance. Although
the retinal-hypothalamic pathway is relaying information about the
new day-night periodicity to the biological clock in the SCN, it usually
takes several days for the body’s internal clock to synch with the new
time zone.
Studies of human sleep are conducted in laboratories where sleeping
humans can be monitored. Such studies often measure three kinds
of physiological activity: brain activity using electroencephalography
(EEG), activity of muscles throughout the body using electromyogra-
phy, and movement specifically of the eyes using electrooculography.
One of the striking findings from these kinds of measurements is that
during certain times of the night we tend to move our eyes around
quite a bit, while during other times the eyes move very little. In
mammals, including humans, sleep is broadly divisible into two sig-
nature stages, that in which rapid eye movements take place (REM)
and that in which such eye movements tend not to occur (non-REM or
NREM).