Interval therapy is the concept of intermittent applications with drug-free intervals, based on the observation that in many cases L-tryptophan has the best effects on disturbed sleep during the drug-free interval after-short-term application. This concept was formulated as an experimental hypothesis to be tested in the sleep laboratory in a predictive, double-blind design, comparing a 4-night placebo period following repetitive 3 X 2 g L-tryptophan application with baseline. All patients, severe chronic insomniacs, 5 males and 3 females (mean age 38.4 years) improved significantly at the predefined level of 0.05. Analyses of the polygraphic recordings proved highly significant sleep improvements in the parameters that are indicators of insomnia. No side effects were seen. It can be concluded that the interval therapy with L-tryptophan is a potent treatment for chronic primary insomnia.