Why is play fun? Rat studies have not identified any particular brain area that seems to be essential
for play, which involves neural populations in many brain areas, as does the emotional processing
described in chapter 5. Play can be inhibited by environmental manipulations that evoke negative
emotional states such as fear, anger, and separation, as well as by bodily disturbances such as hunger
and illness. Play produces widespread release of opioids in the nervous system and can be modulated
by opioids: low doses of morphine can increase play in rats, and opiate antagonists can reduce it.
Activation of serotonin and noradrenaline systems reduces play, and so does blockade of dopamine
systems. A full account of the neurophysiology of play remains to be developed, but there is already
enough evidence to suggest that play is fun because of its neurochemical effects. The studies to date
mostly concern rats, but the relevant neural circuitry and biochemistry are very similar in humans. I
described in chapter 5 the evidence that people enjoy recreational drugs such as alcohol because of
their effects on various neurochemical pathways, including ones involving dopamine and endogenous
opioids.
Little research seems to have been done on the neurochemical reasons why people enjoy playing
sports, but the emotional side of the thrill of victory and the agony of defeat is obvious. Much like
challenging work, sports clearly involve setting goals and striving to accomplish them. Hence the
pleasure associated with winning a game or having a good personal performance likely emanates
from the same neural process of goal satisfaction that can make work satisfying. In addition, exercise
can stimulate the production of endorphins, natural opioids in the brain that reduce pain and produce
feelings of well-being.