Some governments reduce the sentences of prisoners in exchange for their willingness to participate in self-improvement activities or to be of service to the community. In Brazil, for example, prisoners can reduce their sentences by reading books and then writing book reports. Another way for a Brazilian prisoner to reduce his or her sentence is by riding a stationary bicycle that is hooked up to a generator. The power generated charges streetlight batteries.
More about reducing prison sentences:
The federal prison system in the US no longer offers parole, but it does offer prisoners a slight decrease in sentences if the prisoners behave well while incarcerated. The federal prison system also provides larger reductions to prisoners who have been approved for a 500-hour residential drug treatment program.
Some prisoners can get a reduction in their sentences for reporting statements made by other prisoners to the authorities. If the authorities can use the information to solve a crime or successfully prosecute another criminal, the person who reported the statements might receive a sentence reduction or some improvement in his or her conditions.
In the US, some state governments have passed laws to restrict giving time off for good behavior to prisoners who are incarcerated for violent crimes.