Doctors Answers (3)
There are many causes for sleeping 15 or more hours day. Causes range from genetic disorders and brain rumors to medication effect and sleep disordered breathing. You did not give your daughter's age but it sounds as though she has had some periods of normal sleep suggesting that her excessive sleep may be cyclic and return to normal off and on. This could be hormonal in origin. Long sleeping is not necessarily associated with excessive daytime sleepiness which is an important issue. Further evaluation may also include an MRI of the brain. I would recommend seeing a board certified sleep neurologist who is also boarded in sleep medicine.
If she is a newborn that maybe normal. As we age, our sleep needs reduce. By adult life most requirement is average 7.5 to 8 hours. If you are suspicious you can consult with a sleep physician for assurance.
Not knowing her present age I can't be absolutely certain that this amount of sleep isn't normal. If she is between 2-18 months old this may be near normal if some of the sleep time is from daytime naps. By age 5 naps are usually abandoned and the toddler is sleeping 10-11 hours per day. Having to crawl instead of walk may lead to more energy expenditures and fatigue during the day because of this condition.