Sleeping too long on regular basis tends to increase the risks of heart disease, stroke, diabetes, and can cause death according to studies conducted over the years. Too much sleep is considered more than nine hours.
The most common cause of sleeping too long is lack of sleep the night before, or cumulatively during the week. Some other negative effects of sleeping too long include fatigue (oh, the irony!), weight gain, headaches, premature aging, back pain, circadian rhythm disorder, and depression, among others.
For people who suffer from a disorder known as hypersomnia, sleeping for too long is indeed a medical disorder. His condition causes people to suffer from extreme sleepiness, which is rarely relieved by napping. Hypersomnia also makes people who suffer it to sleep for unusually long during the night.
By sleeping longer than nine hours on regular basis, we tend to throw off our regular circadian rhythm. Scientists also speculate that sleeping too much can affect neurotransmitters in the brain, especially serotonin.