I believe that you can sometimes get vivid dreams if you are taking mega doses of an amino acid profile that contains a lot of Tryptophan - which is a precursor for serotonin. When I was at the top of my training I was into all manner of super vivid dreams and heightened awareness and cognition. I liked it actually. But I think I had only a single wet dream in my entire life when I was around 21 and it occurred a moment after being startled awake from an intense erotic dream. Nothing since.
Anyway, Tryptophan, has some unusual properties that I will list in a moment. But it also is induced into the brain by carb loading. When large amounts of carbohydrates are eaten, the body secretes large amounts of the hormone insulin to lower the ensuing high blood sugar. The insulin also clears from the blood most of the 5 amino acids that compete with tryptophan for a ride to the brain. Tryptophan then has the "bus" all to itself, allowing more tryptophan to reach the brain. This is why many people eat large amounts of carbohydrates like bread, cakes, pies, ice cream, chips, pizza, candy etc., especially when they are feeling depressed, stressed or anxious since it produces a drug like calming effect.
Studies with humans and animals conducted over the past 30 years show that serotonin nerve circuits promote feelings of well being, calm, personal security, relaxation, confidence and concentration. Serotonin neural circuits also help counterbalance the tendency of brain dopamine and noradrenaline circuits to encourage over-arousal, fear, anger, tension, aggression, violence, obsessive-compulsive actions, over-eating (especially carbohydrates), anxiety and sleep disturbances.
So basically if you are taking amino supplements or using carbs to spike your insulin you may be getting an OD of Tryptophan that is causing you to get into more deep sleep and perhaps activating more creative and imaginative energies
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OD