Results from several clinical studies show that orally administered melatonin has low bioavailability and a very short half-life. Phenyl capsaicin, a synthetic analogue of capsaicin, might increase its bioavailability by inhibiting the enzymes involved in its hepatic metabolism.

Thus, the hypothesis of the present study is that the administration of melatonin supplement with phenyl capsaicin presents greater bioavailability than a melatonin supplement that does not contain phenyl capsaicin.

Melatonin is an endogenous indolamine that regulates many physiological functions such as reproduction, temperature, mood, bone growth or the immune system. However, since its production is closely related to the light/dark cycle, melatonin is considered one of the main chronobiotic agents that modulates circadian rhythms.

For this reason, in recent years there has been increased interest in the exogenous use of melatonin to address problems of insomnia and circadian rhythm disorders such as jet lag syndrome or shift work. Although many studies have demonstrated the effectiveness of melatonin in treating sleep disorders, pharmacokinetic studies show that it has poor oral bioavailability and a very short half-life. So, new strategies and studies are necessary to increase the low bioavailability of melatonin.

In this context, it has been shown that phenyl capsaicin, a synthetic analogue of capsaicin, might increase melatonin's bioavailability by inhibiting Cytochrome P450 liver enzymes, which are involved in its metabolism. Therefore, the main objective of this study is to quantify and compare the oral bioavailability between a melatonin supplement with phenyl capsaicin and another melatonin supplement that does not contain phenyl capsaicin.

The secondary objectives of the study are to determine the pharmacokinetic parameters:

• Maximum plasma concentration (Cmax).
• Time for maximum plasma concentration (Tmax).
• Half-life (T1/2).
• Area Under the Curve (AUC 0-inf) of plasma melatonin levels

During the study there will be 3 visits: a preselection visit (V0), a visit for the first postprandial study (V1) and after one week washing period, a visit for the second postprandial study (V2).

Inclusion Criteria:

• Men and women between 18 and 65 years old.
• Sign the informed consent form.
• Know how to read, write and speak Catalan or Spanish.

Exclusion Criteria:

• Take supplements or multivitamin supplements or phytotherapeutic products that interfere with the treatment under study up to 30 days before the start of the study (e.g. L-Tryptophan or melatonin)
• Present intolerances and/or food allergies related to melatonin, phenyl capsaicin, microcrystalline cellulose or silicon dioxide .
• Be a smoker.
• Having received antibiotic treatment up to 30 days before the start of the study.
• Present values of body mass index ≤ 18kg/m^2 or ≥ 35 kg/m^2.
• Present some chronic disease with clinical manifestations: coronary heart disease, cardiovascular disease, diabetes mellitus, hypertension, ulcerative colitis, celiac disease, Crohn's disease, chronic kidney disease, cancer, benign prostatic hyperplasia, autoimmune diseases (such as fibromyalgia), respiratory and/or gastrointestinal diseases that may compromise the absorption of the compound.
• Clinical history of anemia.
• Being pregnant or intending to became pregnant.
• Be in breastfeeding period.
• Being unable to follow the study guidelines.
• Participate in or have participated in a clinical trial or nutritional intervention study in the last 30 days before inclusion in the study.