Benefits
Less time awake after sleep onset at about 1 g a day or more
A meta-analysis of tryptophan sleep studies found that supplementation shortened time awake after first falling asleep, with a larger effect at 1 g a day or more, but did not change the other sleep measures, including time to fall asleep. In a later placebo-controlled crossover trial in 98 adults, 1 g a day for a week improved sleep efficiency and time awake measured by wrist actigraphy.
Mood ratings in healthy adults
A review of 11 randomized trials in healthy adults found that 4 showed less negative feeling or more positive mood with tryptophan at 0.14 to 3 g a day, and none showed an effect on aggressive feelings. An older review judged the evidence for low mood to be small, dated and mostly not placebo controlled. Tryptophan is not a treatment for depression.
Premenstrual mood ratings in one placebo-controlled trial
In a randomized trial in 71 women with premenstrual dysphoric disorder, a diagnosed condition, 6 g a day of L-tryptophan taken from ovulation to the third day of menstruation for three cycles reduced a combined rating of dysphoria, mood swings, tension and irritability more than placebo. It is a single older trial at a research dose far above typical supplement amounts.
Mechanism of action
Serotonin synthesis via 5-HTP
Tryptophan hydroxylase converts tryptophan to 5-hydroxytryptophan (5-HTP), which is then converted to serotonin. Serotonin has roles in mood, anxiety, sleep, cognitive function and appetite.
Melatonin synthesis
In the pineal gland serotonin is converted to melatonin, so the same pathway that makes serotonin also supplies melatonin.
Kynurenine pathway competition
Much of the body's tryptophan is metabolized through the kynurenine pathway rather than to serotonin, and about 95 percent of serotonin is made and used outside the brain. It has been proposed that extra tryptophan keeps more substrate available for serotonin, but the studies cited here did not test this.
Clinical trials
Randomized, double-blind, placebo-controlled crossover trial: one week of tryptophan 1,000 mg a day and one week of placebo, with sleep measured by diary and wrist actigraphy. (van Dalfsen et al. 2019, J Psychopharmacol)
98 adults aged 20 to 30 (73 women), grouped by serotonin transporter gene variant: 47 with two short alleles and 51 with two long alleles. The authors note the group slept relatively well at baseline.
Tryptophan improved actigraphy-measured sleep efficiency and time awake after sleep onset in both genotype groups. Self-rated sleep quality improved only marginally, and only in the short-allele group. Each treatment period lasted one week.
Randomized, double-blind, placebo-controlled trial of L-tryptophan 6 g a day or placebo, taken for 17 days from ovulation to the third day of menstruation, over three cycles. (Steinberg et al. 1999, Adv Exp Med Biol)
71 women with premenstrual dysphoric disorder (37 tryptophan, 34 placebo), a diagnosed condition rather than everyday premenstrual discomfort.
On visual analog mood scales, tryptophan reduced the combined dysphoria, mood swings, tension and irritability ratings more than placebo (p = 0.004). It is a single older trial at a research dose far above typical supplement amounts.