From the record
Real cases, written from the published record. Predict first; the record and its sources come after.
2016 · Seattle
The 8:45 bell
In 2016 Seattle's high schools moved the first bell from 7:50 to 8:45, 55 minutes later. Researchers gave students at two schools wrist monitors that tracked their sleep, the spring before and the spring after.
A worry: teens would just stay up later and gain nothing.
How much more sleep did students get on school nights?
What happened. The median gain was 34 minutes a night. Bedtimes moved a little later, but wake-up moved more. At one school, grades rose and late arrivals and absences fell.
Teens did not use all the extra time to sleep, but they used most of it.
1938 · Mammoth Cave, Kentucky
Kleitman's cave
Nathaniel Kleitman, aged 43, and his student Bruce Richardson, aged 23, lived for 32 days deep in a cave with no daylight. They tried to live on a 28-hour day: 19 hours awake, 9 hours in bed.
Their body temperature, which rises and falls with the body clock, was measured throughout.
Who managed to switch to the 28-hour day?
What happened. Richardson's temperature rhythm shifted toward the 28-hour schedule; Kleitman's stayed close to 24 hours, and he slept badly. The body clock is not a habit you can order around; it can be nudged, mostly by light, but slowly.
Your clock isn't a habit. It's biology, and it can be nudged, not ordered.