Hospital sleep may be disrupted by medical conditions, sleep disorders, or environmental factors the researchers noted, and they suggest intervening to improve the sleep patterns of older rehabilitation patients may help promote recovery.

"We were surprised that the results suggested that it was the excessive daytime sleeping in the rehabilitation facility which was associated with less improvement in their physical functioning," principal investigator Dr. Cathy A. Alessi of the VA Greater Los Angeles Healthcare System and the University of California, Los Angeles, David Geffen School of Medicine said in a statement.

"We were also surprised by how long this effect lasted. For up to three months later, more sleeping during the daytime while they were in the rehabilitation facility was still related to their physical functioning after being discharged."

The study, published in the journal Sleep, monitored the sleep of 245 adults with an average age of 80.6 years admitted to one of two study sites for rehabilitation -- for conditions such as an orthopedic problem or a heart attack. The participants were found to sleep an objectively measured daily average of 2.1 hours during the daytime.