Why does waking at 3 a.m. feel so different from waking at 7, even after the same amount of sleep? This episode explains what makes a rhythm truly circadian. In 1918 J.S. Szymanski kept animals in constant darkness and found they still held a 24-hour cycle, and in 1954 Colin Pittendrigh showed that the clock timing fruit fly hatching kept pace even in the cold. From those experiments come the rules: a circadian clock must be free-running, temperature compensated and entrainable by a zeitgeber, the German term for a time giver such as morning light reaching the suprachiasmatic nucleus through special light-sensing ganglion cells in the eye.

The story then goes down to single cells. Gene Block’s work on isolated mollusk neurons showed the clock is cell-autonomous, and the 2017 Nobel Prize recognized the PER and TIM feedback loop that works like a molecular hourglass. The hosts explain why a 2 a.m. meal can reset the liver’s clock while the brain stays on night time, how monarch butterflies and blind mole rats use their clocks, and what happens to mice and hamsters whose clocks are broken or mismatched.

• Monarch butterflies that fly the wrong way when the clocks in their antennae are altered
• Svalbard reindeer that abandon daily rhythms through months of constant light or dark
• Mice with deleted clock genes that overeat and become obese, and the leptin signal that stops getting through
• Tau mutant hamsters with 22-hour clocks forced into a 24-hour day and the heart and kidney disease that followed
• Chronotherapy research on timing ACE inhibitors at bedtime and short-acting statins in the evening