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Amino Acids: Does L-Tryptophan Make You Sleepy?

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What is tryptophan? What does tryptophan do? No amino acid in your body has such a directly noticeable influence on your well-being as tryptophan – the precursor to our happiness and sleep hormones. Eating the right foods provides you with sufficient tryptophan and boosts not only your sleep quality but also your sense of contentment. Find out how!

Will this particular amino acid help treat mental illnesses like depression in the future? Some researchers are certainly resting a lot of hope on the potential of tryptophan. Among other things, tryptophan is involved in producing serotonin, a hormone that is closely linked to mental illness.

But tryptophan has many other tasks, such as enabling us to sleep restfully and protecting our eyes and nerve cells. To optimize these processes in our bodies, we need to consume tryptophan, for example, through the foods we eat every day such as nuts, cereals, and milk.

In this article, you can read about the function of tryptophan and what researchers have discovered about tryptophan deficiency – that is, which symptoms this hormone deficiency triggers. You will also discover more about which foods contain tryptophan and even find out the answer to the age-old question that’s commonly asked around this time of year: What in turkey makes you sleepy?”

What Is Tryptophan?

Tryptophan (L-tryptophan) is an essential amino acid – that is, an amino acid that you have to consume through your diet, as your body cannot produce it itself. Compared with the other essential amino acids, your body only needs small amounts of tryptophan.[1]

In their natural form, amino acids are found in two different forms with different functions – namely, labeled as L (left-handed) and D (right-handed). In the human body, it is mainly the L-amino acids that play an important role.

Is Tryptophan Good for the Brain?

Tryptophan is converted into the happiness hormone serotonin in both your gut and your nervous system.

Before tryptophan enters the nervous system, it must pass through the blood–brain barrier. This barrier separates the bloodstream from the nervous system, so that only certain chemical compounds can enter our sensitive nervous system. Since only a limited number of substances can pass through the barrier, amino acids compete with each other – they prevent each other from crossing the barrier.[1]

happy woman stretching in bed

What Does Tryptophan Do?

All amino acids serve to build new proteins in the body. Apart from this, tryptophan forms kynurenine in our bodies; this is another amino acid that has an antioxidant effect and activates our immune system. In addition, kynurenine is said to protect our eyes from damage caused by UV radiation from sunlight. Your body produces neurotransmitters such as adrenaline or endorphins from tryptophan, which mediate information between nerve cells.

Tryptophan is also the precursor to other important hormones and vitamins, namely:

  • the happiness hormone serotonin
  • the sleep hormone melatonin
  • vitamin B3 (nicotinamide and nicotinic acid) – to produce energy, so that all cells can carry out their tasks and help produce fat and hormones

    Biogenic amines are decarboxylation products of amino acids; tryptamine is produced from tryptophan. This biogenic amine influences the activity of serotonin and melatonin in our bodies.[1, 4]

    Did you know that three percent of the tryptophan that you consume through your diet is converted by your body into serotonin, which, in turn, is used to produce melatonin? More than 90 percent of the serotonin in the body is produced with the help your gut bacteria, also known as your gut microbiome. However, serotonin from your gut cannot reach your nervous system and therefore can only carry out its function in your gut. Among other things, serotonin stimulates digestion.[2, 3]

    It’s not only serotonin that stimulates digestion: Other gut health supplements, such as probiotics can help ensure a happy and healthy gut.

    What Is Tryptophan Intolerance?

    Tryptophan intolerance or tryptophan malabsorption disorder is a congenital metabolic disorder, whereby the body is not able to utilize tryptophan. Instead, the amino acid is converted into the chemical compound indican, which is excreted in the urine.

    As soon as indican comes into contact with air, it turns blue. This is why tryptophan intolerance is also called blue diaper syndrome – blue-colored diapers can be used to identify this metabolic disorder in newborns.

    If you have tryptophan intolerance, you are at risk of vitamin B3 deficiency, which is associated with pellagra disease. The following three D symptoms are characteristic of the disease:[1, 4, 5]

    • dermatitis – skin inflammation
    • diarrhea
    • dementia

    The following health problems can occur due to tryptophan intolerance:[5]

    • growth disorders
    • febrile episodes
    • calcium excess
    • vitamin B3 deficiency and pellagra

    A Tryptophan-Rich Diet: What Foods Have Tryptophan in Them?

    Food makes you happy – and tryptophan is simply proof of that. The amino acid is found in numerous foods, which help our body produce the happiness hormone serotonin. A diet rich in tryptophan can improve your mood and reduce the frequency of depressive moods, according to studies.[8]

    What Foods Have Tryptophan in Them?

    Tryptophan is found in a wide variety of foods. Soybeans, turkey, and tuna are particularly rich in tryptophan. The following foods also contain tryptophan:[1,15, 16]

    • oats, whole-grain cereals
    • bananas, prunes
    • milk, cheese, and other milk products
    • peanuts
    • chicken
    • chocolate
    • chicken
    • egg whites
    • fish
    • sunflower, pumpkin, and sesame seeds

    Thanksgiving feast with turkey in the middle of the table

    What in Turkey Makes You Sleepy?

    Rumor has it that it is the tryptophan in turkey that makes everyone sleepy after their Thanksgiving or Christmas dinner. But is it really the tryptophan in turkey that sends us into a post-meal slumber? 

    Scientists actually believe that the sleepiness we experience after a large turkey feast is partly due to our blood flow being directed to our guts to absorb the new macronutrients and micronutrients – instead of to our brains. This makes us feel lethargic and sometimes lightheaded.

    Moreover, the carbohydrates that accompany our Thanksgiving or Christmas turkey are a key component to consider, as they boost tryptophan absorption in our body (read more about this below). If you consider these factors along with the alcohol consumed, the drowsiness we experience after a large turkey dinner is less surprising!

    How Much Tryptophan Does a Person Need?

    According to studies, the recommended daily requirement is 250 to 425 milligrams of tryptophan. Adults have a requirement of 3.5 to 6 milligrams per kilogram of body weight.

    Newborns have a higher requirement of 12 milligrams per kilogram of body weight.[1, 6]

    How Can I Increase My Tryptophan Levels Naturally?

    Amino acids compete to get through the blood-brain barrier. Carbohydrates improve the absorption of L-tryptophan – eating them will boost insulin production, a hormone that will remove other amino acids from the bloodstream. This means that tryptophan no longer has to compete with the other amino acids to get through the blood–brain barrier. 

    In addition to a carbohydrate-rich meal, low-protein foods also help us absorb more tryptophan: Through them, you absorb fewer other amino acids that compete with tryptophan. To further promote absorption, you should get enough exercise and avoid alcohol.

    Tip: Include tryptophan-rich foods in your diet in the morning and evening. In one study, a breakfast and dinner rich in L-tryptophan improved sleep.[1, 7]

    What Is Tryptophan Deficiency?

    A natural tryptophan deficiency is rare, as we take in enough tryptophan through our diet. To study the effects of tryptophan and the consequences of a potential deficiency, such as a serotonin deficiency, researchers have worked with acute tryptophan depletion (ATD), meaning they artificially induce tryptophan deficiency.

    What Is Tryptophan Depletion?

    In tryptophan depletion, test participants first follow a low-tryptophan diet or drink certain amino acid mixtures. These mixtures are composed in such a way that L-tryptophan cannot pass the all-important blood–brain barrier. By using tryptophan depletion, researchers can reduce the production of serotonin as well as melatonin and thus study the consequences of a deficiency.[9-11]

    Studies using tryptophan depletion have found that foods with a high glycemic index boost the absorption of L-tryptophan.[9]

    What Are the Effects of a Serotonin or Tryptophan Deficiency?

    When your body breaks down too much L-tryptophan, a person’s mood can deteriorate. In studies, researchers have observed this effect especially in women. If you have a family history of depression or other mood disorders, a tryptophan deficiency, and therefore a serotonin deficiency, is thought to increase your tendency to experience depressive episodes.[1, 6, 7]

    For people who have suffered from depression in their lives, tryptophan deficiency could trigger another episode of depression.[9] Researchers also observed the following:[1, 6]

    • long-term memory could be impaired,
    • aggression increased in men, and
    • if irritable bowel syndrome was present, bowel movements worsened.

      What Causes Tryptophan Deficiency?

      In addition to tryptophan intolerance, fructose intolerance can interfere with tryptophan absorption, as the body absorbs less of the amino acid. People suffering from Crohn’s disease, a chronic inflammatory bowel disease, also have low tryptophan levels – a reason for this is that the gut’s ability to absorb tryptophan is impaired.[3, 12]

      Tryptophan Supplements: Should I Take Tryptophan or 5-HTP?

      L-tryptophan has been shown to improve sleep quality in studies. As the amino acid promotes the production of melatonin, the sleep hormone, many sleep supplements contain tryptophan or chemical compounds made from tryptophan, such as 5-HTP (5-Hydroxytryptophan).

      5-HTP and L-tryptophan are both considered natural alternatives to the more traditional antidepressants, such as SSRIs. 5-HTP is the chemical byproduct of L-tryptophan and can be converted into serotonin in the body. As 5-HTP is not affected by the presence of other amino acids in the body, it can be absorbed more easily and effectively get through the blood–brain barrier.[18]

      Some studies have investigated the effects of taking SSRIs, such as fluoxetine, and 5-HTP concomitantly to treat depression and anxiety in adult rats. However, data on the effectiveness of this combination is limited, and further experiments need to take place in order to come to a more concrete conclusion.[17]

      Did you know that tryptophan can brighten your mood if you have seasonal depression? Combining three grams of L-tryptophan daily and light therapy had a similar effect to antidepressants prescribed for winter depression.[1]

      As with many supplements and medications, you should discuss taking 5-HTP with your doctor to make sure that it doesn’t react with any other medications that you are taking.

      woman with winter depression looking out the window

      Can I Take Tryptophan for Withdrawal Symptoms?

      If you want to quit smoking, you can try taking tryptophan. Researchers were able to observe in a study that people experience fewer withdrawal symptoms after they stop smoking if they take 50 milligrams of L-tryptophan per kilogram of body weight daily. You should also make sure you eat a diet rich in carbohydrates.[1]

      What Are the Side Effects of Tryptophan?

      The problem with taking L-tryptophan is that the amino acid can not only be converted into serotonin and melatonin, but also into countless other substances. Therefore, it is not always possible to determine what effect L-tryptophan will have on your body. There is a risk of the following side effects:[1]

      • fatigue
      • dizziness
      • nausea
      • diarrhea
      • headache

      Taking L-tryptophan can also interfere with the effect of some medications. If you are taking the following active substances, it is best to discuss tryptophan supplements with your doctor:[1]

      • MAO inhibitors (antidepressants)
      • SSRIs (antidepressants)
      • triptans (for migraines)
      • dextrometorphan (for an irritable cough)

      In 1988 people who took L-tryptophan supplements became ill with eosinophilia-myalgia syndrome – a condition characterized by severe muscle pain. The reason for this was contamination of dietary supplements from a Japanese company. Selling L-tryptophan was therefore banned for a short amount of time. Only after safety precautions were taken in production could the supplements be sold again.[1]

      What Is Tryptophan – at a Glance

      What Is Tryptophan?

      L-tryptophan is an essential amino acid and is found mainly in nuts, cereals, and dairy products. To enter the nervous system, it has to cross the blood–brain barrier, where it competes with other amino acids that are also trying to get through the barrier.

      What Does Tryptophan Do?

      Tryptophan serves as a precursor to certain hormones, including the happiness hormone serotonin and the sleep hormone melatonin. The body also produces messenger substances from tryptophan and the amino acid kynurenine, which has an antioxidant effect and protects the nerve cells. Tryptophan is also the precursor to vitamin B3, an important vitamin for energy metabolism.

      Which Tryptophan Supplement Is the Best?

      Studies have shown that L-tryptophan supplements improve sleep quality by increasing melatonin production. It is also said to be a way to treat winter depression and reduce withdrawal symptoms when quitting smoking.


      [1] Richard D. M., Dawes M. A., Mathias C. W., Acheson A., Hill-Kapturczak N., Dougherty D. M. L-Tryptophan: Basic Metabolic Functions, Behavioral Research and Therapeutic Indications, Int J Tryptophan Res, vol. 2, pp. 45–60, March 2009.

      [2] Bornstein J. C. Serotonin in the Gut: What Does It Do?, Front Neurosci, vol. 6, February 2012.

      [3] Roager H. M., Licht T. R. Microbial tryptophan catabolites in health and disease, Nat Commun, vol. 9, August 2018.

      [4] Cervenka I., Agudelo L. Z., Ruas J. L. Kynurenines: Tryptophan’s metabolites in exercise, inflammation, and mental health, Science, vol. 357, no. 6349, July 2017.

      [5] Elmadfa I., Leitzmann C., Ernährung des Menschen, 6th edition, Stuttgart: Eugen Ulmer KG, 2019.

      [6] Palego L., Betti L., Rossi A., Giannaccini G. Tryptophan Biochemistry: Structural, Nutritional, Metabolic, and Medical Aspects in Humans, J Amino Acids, vol. 2016, 2016.

      [7] Bravo R. Tryptophan-enriched cereal intake improves nocturnal sleep, melatonin, serotonin, and total antioxidant capacity levels and mood in elderly humans, Age (Dordr), vol. 35(4), pp. 1277–1285, August 2013.

      [8] Lindseth G., Helland B., Caspers J. The Effects of Dietary Tryptophan on Affective Disorders, Arch Psychiatr Nurs, vol. 29(2), pp. 102–107, April 2015.

      [9] Jenkins T. A., Nguyen J. C. D., Polglaze K. E., Bertrand P. P. Influence of Tryptophan and Serotonin on Mood and Cognition with a Possible Role of the Gut-Brain Axis, Nutrients, vol. 8(1), January 2016.

      [10] Linden M. et. al. Dietary tryptophan depletion in humans using a simplified two amino acid formula – a pilot study, Food Nutr Res, vol. 60, December 2016.

      [11] Neumeister A., Praschak-Rieder N., Heßelmann B., Tauscher J., Kasper S. Der TryptophandepletionstestGrundlagen und klinische Relevanz, Nervenarzt, vol. 68(7), pp. 556–562, July 1997.

      [12] Ledochowski M., Widner B., Murr C., Sperner-Unterweger B., Fuchs D., Fructose malabsorption is associated with decreased plasma tryptophan, Scand. J. Gastroenterol., vol. 36(4), pp. 367–371, April 2001.

      [13] Fukushige H. et al. Effects of tryptophan-rich breakfast and light exposure during the daytime on melatonin secretion at night, J Physiol Anthropol, vol. 33, p. 33, November 2014.

      [14] Hornig, M. Why Does Tryptophan in Turkey Make Us Sleepy? Columbia Mail School of Public Health, available at, accessed on November 18, 2021.

      [15] “Tryptophan,”, available at, accessed on November 17, 2021.

      [16] “Tryptophan,” MedlinePlus, available at, accessed on November 18, 2021.

      [17] Hansen F., de Oliveira D. L., Amaral F. U., et al. “Effects of chronic administration of tryptophan with or without concomitant fluoxetine in depression-related and anxiety-like behaviors on adult rat,” Neurosci Lett., 2011, vol. 499(2), pp. 59–63, doi:10.1016/j.neulet.2011.05.032.

      [18] Birdsall T. C. 5-Hydroxytryptophan: a clinically-effective serotonin precursor, Altern Med Rev., 1998, vol. 3(4), pp. 271-280.

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