When older mice don't get enough vitamin D, giving them quercetin (a plant compound) changes some of their body clock genes and increases a key clock protein at a specific time of day—but if the mice have enough vitamin D, quercetin doesn't do anything noticeable.

When older mice don't get enough vitamin D, giving them quercetin (a plant compound) makes a key body clock gene less active at a specific time of day—but only when they're also low on vitamin D. If they have enough vitamin D or don't get quercetin, this doesn't happen.

When middle-aged mice don't get enough vitamin D, their body's internal clock gets mixed up: some clock proteins drop, but one important one goes up—even though the genes making them didn't change much. It's like the recipe book says 'make more,' but the kitchen isn't following it.

When middle-aged mice don't get enough vitamin D, their blood sugar control gets worse in the evening, but giving them quercetin—a plant compound—doesn't help fix it.