There are 4 different sleep stages that humans cycle through at night. Deep sleep is one of those stages, and is typically seen in the first half of the night, at least in adults. Sleep researchers do not know exactly how deep sleep is produced, but it's thought to involve the neurotransmitter GABA, which stands for gamma-aminobutyric acid.
Here is a report of a study involving stimulation of a deep part of the brain (the brainstem) that turns on deep sleep. The technique used to stimulate this area of the brain is different than other methods involving electrical stimulation of the cells in the brainstem felt to be responsible for deep sleep. With electrical stimulation, the problem is that the electrode stimulates everything it touches, including surrounding areas it did not come into contact with. As stated in the article, electrically stimulating cells was like using a sledgehammer, when what was needed was a scalpel. In this study, researchers inserted a harmless virus that incorporated itself precisely into the very cells the researchers were trying to study. The cells were then remotely "turned on" and the animals went right into deep sleep.
This is exciting research as it points to the possibility of developing therapies that enhance deep sleep by targeting only certain brain areas. Hopefully this will lead to treatments with more specific modes of action and less side effects.