Exogenous zeitgebers and endogenous pacemakers

Endogenous pacemakers

Internal body clocks

Exogenous zeitgebers

External cues affecting biological rhythms

Suprachiasmatic Nucleus

• Small bundle of cells in the hypothalamus

• Controls the circadian rhythm

Decoursey et al

Destroyed the SCN in 30 chipmunks and found that the chipmunks no longer had a sleep/wake cycle. Many were killed, suggesting an evolutionary benefit to the SCN

Ralph et al.

Bred hamsters with 20 hours sleep/wake cycle and implanted their SCN cells into normal hamsters. The normal hamsters also developed 20 hour sleep/wake cycles

Light

Light acts as an exogenous zeitgeber

Campbell and Murphy

Shone lights on participants’ knees and was able to affect their sleep/wake cycle by up to 3 hours

Social cues

Social cues such as meal and bedtimes act as exogenous zeitgebers

Miles et al

Case study of a blind patient with a circadian rhythm of 24.9 hours which was not affected by factors like social cues