Think of an adult you know who sleeps 10 hours a day. Is that a healthy person? Probably not. Experts like Matthew Walker, the director of the sleep and neuroimaging lab at the University of California, Berkeley, have discussed how various diseases and chronic ailments might prompt someone to sleep more yet also put them at higher risk of death. He discusses studies like this one in his book, Why We Sleep, in which he points out that "should you explore those studies in detail, you learn that the causes of death in individuals sleeping nine hours or longer include infection (e.g., pneumonia) and immune-activating cancers." Serious illnesses like these prompt a person to sleep more, he points out, and thus "the illusion created is that too much sleep leads to an early death, rather than the more tenable conclusion that the sickness was just too much despite all efforts to the contrary from the beneficial sleep extension."