MTN *****ING US ONCE AGAIN

Once again mtn is *****ing data. I have a data contract for 20gb day and 20gb night which runs from 12 to 6am in the morning. I now get a message that they are changing this to 1am to 5am so we loose 2 hours of night time data unless you never sleep at night. This was not explained when I signed the contract in October last year. Is this legal?