- (B) The siblings have different circadian rhythms, approximately 24-hour cycles in biological and
behavioral processes including those that regulate alertness, body temperature, and heart rate.
Sleep cycles are an example of an ultradian rhythm, a cycle that happens multiple times a day. The
sleep cycle is approximately 90 minutes long, and we cycle through it several times over the
course of a night’s sleep. Even though the siblings may differ in terms of their daily activities and
personalities, these are unlikely to account for their consistent differences in peak alertness time.
marvins-underground-k-12
(Marvins-Underground-K-12)
#1