In general, the Catholic Church doesn't have many specifics when it comes to fasting. Lent includes two primary days of fasting, Ash Wednesday and Good Friday, while also instructing Catholics to abstain from meat on every Friday during Lent.
This is meant to provide Catholics with a certain amount of freedom to choose those disciplines that help them the most in their relationship with God. Often such Lenten disciplines are chosen with the help of a spiritual director or trusted priest.
St. Thomas Aquinas wrote more specific guidelines in his Summa Theologiae, first reflecting on the natural rhythms of the body as he saw them at his period in history.
The ninth hour
With this in mind, St. Thomas Aquinas argues that extending one's fast another 3 hours -- to the "ninth hour" (3:00 p.m.) -- can be a suitable sacrificial act.
This type of fasting isn't a requirement of the Catholic faith, or something that everyone should adopt immediately into their life. Just as with any change in diet, you should consult your physician to see if your body can handle such a practice.
Aquinas' suggestion reminds us that Lenten disciplines, such as fasting, should "hurt" a little bit. If it does, it unites us more closely to Jesus' Passion, and reminds us of the great sacrifice that he made out of love for us.
