Being overweight doesn t directly cause excessive sleep but there may be a link: 1. Being overweight may exacerbate conditions such as sleep apnoea that in turn may cause day time sleepiness. Though more common in men, a woman may also have sleep apnea. If she snores during sleep, then this is a possibility. A sleep study (polysomnograph) helps in diagnosis. 2. Sometimes, condition such as hypothyroidism may cause both weight gain and excessive sleep. A simple blood test helps in diagnosis of this condition. Some viral or bacterial infections, depression, and some illnesses such as anemia, congestive heart failure may lead to excessive day time sleepiness. If her routine medical check up doesn t reveal a cause, you should take her to a specialist in sleep disorders. If she has other symptoms such as sadness, she should consult a psychiatrist. The doctor may decide to prescribe stimulant medicines such as modafinil or armodafinil or Adderall. I hope this answer is helpful.