I expect the biggest thing is stress... If you want something constantly and strongly and are not getting it, that will have an effect on your mental health and probably, as an extension of that, your physical health as well. The extent to which is does this most likely depends on how stressed a person is getting by the situation. If it makes them depressed, gives them headaches, distracts their concentration... then quite possibly it could affect their sleep, efficiency etc.

Where masturbation vs sex is concerned, while the physical sensation might be perfectly pleasurable, there are mental benefits from sex if a person desires it- emotional benefits, bonding connections, a feeling of success and desirability... so there are things that sex can give a person that masturbation cannot.