Serving short sentences prevents criminals from breaking the cycle of reoffending, a new study suggests.
The report by the Scottish Centre for Crime and Justice Research said most offenders wanted to become law-abiding citizens.
But it said short sentences were an "obstacle" to rehabilitation.
The report found the typical short-sentence prisoner had a serious, long-term issue with drugs and alcohol.
It said short sentences were often used by prisoners as a drying out session.
While this allowed some sober time for reflection, it rarely developed into a sustained period of staying off drugs, the report said.
It concluded that using prison as a detox facility was an "expensive proposition".