Can some foods really make you sleep ?

• By Claudia Hammond
• Common wisdom holds that certain meals can make you drowsy, but is that true? Claudia Hammond explains.

Roast turkey tends to get most of the blame for causing drowsiness. And anyone who has had their fill at Thanksgiving or Christmas is probably familiar with the sleepy feeling that arrives afterwards. But is it fair to blame the turkey?

The reason it’s a prime candidate for sleepiness is that it contains the substance L-tryptophan. But other foods have more. Sea lion kidney, for example, or if you are looking for something more everyday, egg white, cod or pork chops. I can’t think of a recipe that combines all these ingredients, but even if you were to succeed in finding one, it wouldn’t necessarily have soporific effects.

Sea lion kidney, egg white, cod and pork chops all contain more L-tryptophan than turkey

The idea that some meals make you drowsier than others may not be entirely true. To understand why, you need to understand how the body and brain absorb nutrients from food.

L-tryptophan is an essential amino acid. The body can’t produce it and must obtain it from the diet in order to use it as a building block to make serotonin. This is the neurotransmitter we sometime hear associated with happiness, but it also makes fruit flies sleepy and might promote slow-wave sleep in humans.

This would suggest that taking tryptophan can help you sleep and indeed when it’s taken as a pharmaceutical preparation it can treat insomnia, according to a review of studies conducted before 1986. Fewer such studies have been conducted in the intervening years, but in 2002 one study added some extra support: it showed that decreasing tryptophan levels worsens sleep.

• READ MORE: What foods can I eat for a good night’s sleep?

The substance’s effects in these studies are more complex than first appears, however, because tryptophan from the diet is not the same as from a pharmaceutical preparation: in food, it’s not alone. The protein in a meal also contains large neutral amino acids or LNAAs and these compete with tryptophan to get across the blood-brain barrier. For food containing tryptophan to get the chance to produce serotonin, you not only need to eat it on an empty stomach where it doesn’t compete with the other food you’ve eaten, but you need a food that doesn’t contain other amino acids, which doesn’t happen.

Tryptophan in medicinal form can help you sleep, but it's different in food (Credit: Getty Images)

To work, you need to eat it on an empty stomach and without food that contains other amino acids  

But there is a trick which could help the tryptophan to get through. In a study conducted with people with insomnia in Ontario, Canada, volunteers were given food bars containing butternut squash seeds. These are high in tryptophan. When the sugar dextrose was added to the bars, sleep was improved. (Nice detail in this study: by chance, a pair of identical twins both responded to adverts to take part, unaware that their twin was doing the same).

The reason dextrose made a difference is that sweet carbohydrates cause the secretion of insulin which encourages other amino acids to be absorbed in tissues, leaving the tryptophan more likely to succeed in crossing the blood-brain barrier.

But even if you choose the right combination of foods, it won’t necessarily work for you. The 40 controlled studies of supplements of L-tryptophan conducted before 1982 showed that people did feel  subjectively more sleepy and took less time to fall asleep, but there was less evidence that it could increase the number of hours people slept for, especially if they didn’t suffer from insomnia. And if the insomnia was severe, it had little impact. So it might only work for mild insomnia.

So is the turkey on Christmas Day or at Thanksgiving making you feel sleepy? Not on its own, but possibly in combination with other foods. However, there could be completely different reasons for feeling tired – getting work finished off before the holidays, cooking for large numbers of people and maybe burning the candle at both ends with parties too. So you might well have started the day already exhausted. Add to that indulging in a huge meal and maybe some alcohol and then, even if L-tryptophan didn’t beat the other amino acids in the race to your brain, no wonder you feel like a little snooze.

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