This randomized cross-over double-blinded trial aims to investigate the health effect of oral l-tryptophan supplementation on workers who rotate night shift work, and to examine the influences of different frequencies and timings of supplementation on the health effects.

This study aims to investigate the health effect of oral l-tryptophan supplementation on workers who rotate night shift work, and to examine the influences of different frequencies and timings of supplementation on the health effects. Night shift work affects melatonin and cortisol variation, insomnia, mood issues, and metabolic syndrome. Short-term tryptophan supplementation was shown to improve melatonin, sleep quality, and cortisol response to acute stress. Little is known whether a longer term of longer-term tryptophan supplementation would alleviate the health impacts of night shift work. Nevertheless, melatonin and cortisol have their normal circadian patterns. Nothing is known about what temporal structure of tryptophan supplement would maximize its health effects on night shift workers melatonin and cortisol levels. We will conduct a double-blind cross-over randomized controlled trial to examine the benefits of oral tryptophan administration (3g/day, t.i.d.) on night shift workers' health outcomes: melatonin and cortisol levels, sleep quality, moods and work performance. We expect to reveal the health effect of tryptophan supplementation in night shift workers by using sophisticated study design.

Inclusion criteria:

• Age: adults >=20 y/o
• Adults who had night shift schedule >1 month before enrollment, and will have similar night shift schedule in the following 2 months. (Eligible night shift schedule: >1 day of evening or night shift in a week)
• People who will not have plans to travel to another time zone in the following 2-3 months
• People who will plan to stay in the same job position for the following 3 months
• People who have no experience in taking tryptophan or melatonin supplement in the past month
• Body weight >= 50kg and <=80kg
• Non-pregnant, non-breastfeeding, not taking contraceptives
• Not under treatment of steroids, not taking sleeping pills

Exclusion criteria:

• diagnosis of renal or liver diseases
• diagnosis of Cushing's Disease, Addison's disease
• psychiatric diagnosis, such as depression and anxiety disorders
• neurological diseases such as dementia, epilepsy, Parkinson's disease

The participants did not provide agreement to the researchers.