Circadian rhythm (and what reward does to it)

The roughly 24-hour cycle that physiology and gene expression run on. It arrives in this spoke through Lewis et al. 2021, whose actual subject is not reward in general but the two-way coupling between reward and the clock — and it is the first subject of its kind anywhere in this hub.

The parts

The SCN — the suprachiasmatic nucleus of the hypothalamus, "the master regulator of biological clocks." Light is its primary zeitgeber (time-giver), the German term the field kept.

Clock genes — Clock, Bmal1, Npas2, Per1, Per2, Rev-ERBα. Rhythmic gene expression is found throughout the central nervous system, and the chapter's opening point is that some of it is independent of the SCN. A brain region can keep time without being told.

The loop

The interesting claim is not that the clock gates behaviour. It is that behaviour sets the clock.

Clock → reward. Clock and Rev-ERBα suppress tyrosine hydroxylase, so dopamine synthesis is timed; NPAS2, BMAL1 and PER2 control the enzyme that clears it. Lose either circadian gene and rhythmic TH expression falls apart.

Reward → clock. Dopamine receptor activation changes clock-gene expression in the striatum. D1R activation in the SCN itself shifts circadian phase, and a direct VTA→SCN connection has been described. The SCN reaches back to the VTA indirectly, through the medial preoptic nucleus.

So the non-photic time-givers are the rewarding activities: feeding, exercise, sex, substance use and social interaction. Do them at an unusual hour often enough and the argument is that you have moved a clock, not just lost sleep.

Where the coupling is claimed to matter

Obesity by timing, not amount. Mice lacking Bmal1 or Per2, and ClockΔ19 mutants, become obese from eating equally across day and night. A Per1 mutation shifts intake into the rest phase and obesity follows. Human hunger has its own rhythm — lowest at 8am, highest at 8pm, whatever the food (Scheer et al. 2013).
A food clock nobody has located. Food anticipation survives SCN lesions and deletion of Bmal1, Per1 and Per2. The chapter's candidates are elsewhere in the hypothalamus and in the striatum, and it says plainly that the anatomy and mechanism "remain elusive."
Addiction. People with addiction have badly disrupted rhythms; evening chronotypes are over-represented in depression, insomnia and substance abuse; and drug use produces lasting circadian changes that outlive the drug. Which way the causation runs is not established.
Screens. Blue light suppresses melatonin and shifts timing; the chapter also proposes that nighttime device use disrupts rhythm through striatal dopamine release from the social reward itself, which is a mechanism claim resting on associational evidence.

The clock has a behavioural night shift (2026-09-19)

Everything above is chronobiology at the molecular scale. Tubbs et al. 2022 attach behaviour and a clinic to the same clock, and the claim is that circadian phase is a risk variable in its own right — separable, in principle, from how long you have been awake.

What moves across the 24 hours, on their reading of the literature:

Positive affect bottoms out at 1am–4am; negative affect, flat by day, climbs sharply at night.
Striatal reward activation is greatest in the afternoon.
Behavioral inhibition is maximal in the circadian day and declines through the night.

This page's own account already supplies the molecular floor under that: dopamine peaks in the active phase because TH is high and MAOA low at the same hour. Tubbs et al. add that sleep deprivation reduces available D2 receptors in the ventral striatum — the inhibitory half of the pair reward-system says must be signalled together.

The consequence, set out in mind-after-midnight-hypothesis, is that the risks of being awake at 3am — suicide, overdose, impulsive decisions — may be about when you are awake rather than only how long. Nobody has separated the two experimentally; that is the point of mind-after-midnight-nimh-grant.

The seam with this page's founding source. lewis-borrelli-reward-2021 argues reward acts on the clock — feeding, drugs and social contact as zeitgebers. Tubbs et al. run the arrow the other way: the clock acting on reward. Neither contradicts the other, and together they are the loop this page opened with, now closed from both ends.

The hedge to keep with it

The chapter's last sentence: "the molecular mechanisms that define dopamine-circadian interactions and their consequences on our health are still in their infancy." Everything above is a coupling that a well-cited review calls immature, in a spoke whose founding note is that popular writing compounds a replication problem. The chronobiology-and-diet literature is exactly the kind that gets compressed into advice.

dopamine · reward-system · lewis-borrelli-reward-2021 · dopamine-nation · mind-after-midnight-hypothesis · tubbs-mind-after-midnight-2022 · synthesis