Healthwatch England put in a Freedom of Information request and found psychiatric patients now spend six fewer days in hospital than in 2012-13.
The data, from 32 mental health trusts, shows average number of "bed days" per patient per year has fallen by 10%.
Healthwatch England says some patients are being discharged too early and without enough support.
Chairwoman Anna Bradley said the findings reflect a troubling picture where patients discharged from hospital often struggle to access crisis treatment at home.
"What is particularly problematic is when people leave institutions, they cannot access the support they need in the community. We have heard of cases where people have taken their own lives.
"Our concern is that people are being lost between hospitals and community services."
[For more of this story, written by Gail Champion, go to http://www.bbc.com/news/health-32782647]
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