Depression after childbirth is called as post-partum depression. This type depression often can lead to severe disability. In addition, several other psychiatric disorders can be precipitated after child birth such as bipolar disorder/obsessive compulsive disorder, etc. Several of these disorders are treatable - almost 70-80% of individuals suffering from depression / anxiety disorders can improve with proper treatment (medications + therapy).
Individuals with depression may sleep longer, however, several medications (lonazep) may also cause sedation. Thus a balance is needed so as to not make patient excessively sedated during the day. This often requires several visits to physician. Sleeping more by itself is not of any harm except that you do not function well during the day. I strongly suggest that you follow-up with psychiatrist regularly for a comprehensive examination and changes in medications.
Additionally, in such cases a good support system (either family or friends) might be beneficial to relieve some stress on the patient. Further, if the patient is able to express her feelings to someone that could help to some extent as well. See if a mental health centre near your location provides any counselling services for depression.