How exactly does eating carbohydrates or a high GI meal close to bedtime help sleep?

It is thought that a carbohydrate-rich or a high-glycemic-index meal can affect sleep through its effects on tryptophan (an essential amino acid) levels.^[1] More specifically, high-glycemic-index carbohydrates cause a rise in insulin levels, which drive circulating large neutral amino acids (LNAAs)^[2] into muscles. Because LNAAs compete with tryptophan for transport across the blood-brain barrier, lower levels of blood LNAAs result in a higher tryptophan-to-LNAA ratio, so more tryptophan can travel to the brain. More tryptophan entering the brain leads to greater production of serotonin,^[3] and, finally, to the secretion of melatonin (a sleep-inducing hormone), resulting in sleepiness.