Which hormone is responsible for regulating the sleep cycle?

Dorothy , You always have melatonin in your body. If you sometimes take it, orally, as a supplement , I suggest you ask your doctor. It is unlikely to do harm. But it is not well absorbed and does not penetrate blood-brain-barrier BBB effectively. You might be wasting money.

I think it's circadian rhythm, the melatonin hormone raises to promote sleep, while several other hormones lower.

I have insomnia so I know all about taking melatonin.

AKA the anti-aging hormone, due to its antioxidant properties.

I sometimes have melatonin