If a rat has been eating lots of sugary treats, and then is given regular food after being hungry, its brain’s serotonin activity goes down—no matter if it was stressed or not.

When stressed rats eat sugary food after being hungry, their brain's serotonin activity goes up—but if they’ve been eating sugary food for a while and then eat normal food, serotonin activity drops.

When rats are stressed and eat sugary food, their brain’s chemical balance changes differently when they’re hungry versus when they eat again—especially in the area linked to pleasure and reward.

Whether a rat is hungry or just ate, and what it’s been eating before, changes the levels of two key brain chemicals in its reward center.