Question

exposure to light affects the bodys production of which helps regulate the sleep - wake cycle
neurons
tryptophan
narcolepsy
melatonin

Explanation:

Melatonin is a hormone produced by the pineal gland in response to darkness. Exposure to light inhibits its production. It plays a key - role in regulating the sleep - wake cycle. Neurons are nerve cells, tryptophan is an amino acid, and narcolepsy is a sleep disorder, none of which are directly related to light - regulated sleep - wake cycle in this context.

Answer:

Melatonin