The Sundays of Lent are part of the Time of Lent. Are they prescribed days of fasting and abstinence?

The only days of fasting during Lent are Ash Wednesday and Good Friday, and the only days of abstinence are Ash Wednesday and the Fridays during Lent.

Now, there are some who say that if you give something up for Lent, you can take Sundays off. Others say that if you take Sundays off you're taking the easy way out. I say that since 'giving something up for Lent' is an optional practice in the first place, you can make up whatever rules you like because it self imposed in the first place, therefore, you get to make up the rules.