Sleep and Tryptophan
Does drinking milk make you sleepy? Yes and no. Protein foods such as milk and milk products contain the sleep-inducing amino acid tryptophan. Having warm milk at bedtime is a good way to work towards reaching the recommended number of servings of Milk and Alternatives each day, and can be a comforting way to unwind. Yet, consuming milk as such is unlikely to induce sleep, as the amount of tryptophan present in milk is too small.
Tryptophan is an amino acid that promotes sleep and is found in small amounts in all protein foods. It is a precursor to the sleep-inducing compounds serotonin (a neurotransmitter), and melatonin (a hormone which also acts as a neurotransmitter). Thus, for tryptophan to have a sedative effect, it needs to enter the brain by crossing the blood-brain barrier. Additionally, since tryptophan uses the same transport system as other amino acids, it has to compete against them to enter the brain.1
Although eating protein foods alone is unlikely to effectively induce sleep due to their insufficient tryptophan content, eating carbohydrates and protein in the same meal makes tryptophan more available to the brain. A large meal, especially one rich in carbohydrates, stimulates the release of insulin. This promotes the clearance of other amino acids from the bloodstream and facilitates the entry of tryptophan into the brain.2 Yet, how much carbohydrate is needed and whether this will induce sleep is uncertain.
On the other hand, some studies suggest that α-lactalbumin, a milk protein with a high tryptophan content, may also lead to improved sleep through an increase in the ratio of tryptophan to other amino acids in the blood.3,4,5 However, these studies looked at α-lactalbumin as a supplement to meals, meaning that more evidence is needed for a definite conclusion. Furthermore, whether the amount present in milk is sufficient to have an effect is unclear.
Consuming milk or milk products alone is probably not going to induce drowsiness or sleepiness, because milk in itself does not contain a sufficient amount of tryptophan. Hence, milk and milk products can be consumed at any time of the day. As well, milk is a nutritious and comforting food that can be included in a routine before going to sleep. Not only can this food choice be a soothing way to end the day and have a good night’s sleep, but it can also help people meet the recommended 2 to 4 servings of Milk and Alternatives they need each day, depending on their age group.
- Silber BY and Schmitt JA. Effects of tryptophan loading on human cognition, mood, and sleep. Neurosci Biobehav Rev 2010;34:387-407.
- Wurtman RJ et al. Effects of normal meals rich in carbohydrates or proteins on plasma tryptophan and tyrosine ratios. Am J Clin Nutr 2003;77:128-132.
- Markus CR et al. The bovine protein alpha-lactalbumin increases the plasma ratio of tryptophan to the other large neutral amino acids, and in vulnerable subjects raises brain serotonin activity, reduces cortisol concentration, and improves mood under stress . Am J Clin Nutr 2000;71:1536-1544.
- Markus CR et al. Evening intake of α-lactalbumin increases plama tryptophan availability and improves morning alertness and brain measure of attention. Am J Clin Nutr 2005;81:1026-1033.
- Minet-Ringuet J et al. A tryptophan-rich protein diet efficiently restores sleep after food deprivation in the rat. Behav Brain Res 2004;152:335-340.