"Day2 inpatient chemo- sleeping a lot, liking new anti nausea med this time, & looking to get home today! #GetWellJen," Arnold said in her Twitter post.
People.com said Arnold's chemotherapy had been postponed for a week due to low blood counts.
The reality TV personality, neo-natologist and mother of two young children said having to wait to start treatment meant she and her family had "a better Xmas."
Celebrity Families of 2014 [PHOTOS]