Serotonin is made from tryptophan, an essential amino acid you can only get from food. To reach the brain, tryptophan has to pass a transporter it shares with several other amino acids – and it is the rarest of them. A carb-rich meal releases insulin, which moves many competitors into muscle, so tryptophan's share at the gate rises. A protein-rich meal does the opposite . The mechanism was first shown in animals ; in people, the shift is real but modest.

At night, serotonin is the raw material for melatonin. In one trial, a high-glycaemic carbohydrate meal eaten four hours before bed shortened the time it took to fall asleep, compared with a low-glycaemic meal .

The serotonin story has limits. A large umbrella review found no consistent evidence that depression is caused by low serotonin . Think of serotonin as one dial among many – not the master switch.

Manuel uses a simple picture. Sitting in a warm bath feels safe. Everyday life drains the tub a little, all the time – and if nothing flows back in, you end up sitting in cold water. Then life pulls the plug: a death, an illness, a separation. Stressful life events are among the strongest triggers of a depressive episode .

You can't control the plug. You can control the taps – and several have solid evidence. Improving diet reduced depressive symptoms in a randomised trial of adults with major depression . Exercise reduces depressive symptoms, with walking, jogging, yoga and strength training all showing benefits . Better sleep leads to better mental health . And antidepressants outperform placebo in acute major depression . If you take one, it is one of your taps: never stop suddenly – taper only with your doctor.