Sometimes the most efficient form of practicing singing doesn’t involve singing at all.
Singing is an athletic endeavor, don’t let anyone tell you any different. We are asking our body to do the same thing over and over again for a desired outcome.
Just like a major league pitcher or an Olympic sprinter, a singer’s body is our instrument. And no matter how good your technique or how healthfully you sing, there comes a point where too much practice singing will fatigue your instrument.
The common wisdom is that a singer should sing for no more then about three hours a day total, and not more then about an hour straight without a break for vocal rest. But every singer is different. It is so important to know your instrument well enough that you are aware of what it feels like as you approach the threshold of overuse so that you can stop singing before you reach it. After that point, the idea of “mental practice” becomes essential.
What is mental practice? Mental practice consists of taking yourself through your music in your mind’s eye (or in this case, ear) in a deliberate and specific way to reinforce your technique and performance intention without doing any actual singing.
Mental Practice Exercise
Guide yourself step by step through a performance virtually in your mind in the following way: Lay down on a comfortable surface with you feet flat on the surface and your 
knees up. Support your head with something sturdy like a book (rather then a pillow which engages sleep). Keep your arms resting alongside your body. Close your eyes and allow for a few cleansing breaths to come in. Then visualize the physical space you will be performing in and put yourself in that space. Hear the intro to your music and breathe at the same moment and with the same kind of open, peaceful breath you would take before the first phrase of the song. Guide yourself through your song in your mind’s eye in real time. Give yourself all of the same technical and emotional directions you would if you were singing. This sets those images and technical reminders into your mind as those moments occur in the song. Feel the musical transitions in your body. When you come to a note or section that is usually difficult for you when singing, give yourself the reminder of the technical tools you can use to ensure those phrases come out successfully. Take yourself through your entire song in this way through to the end. If there is a section of musical interlude in which you don’t sing, don’t skip ahead to the next section you sing. Instead, see yourself through that interlude section, hear that music, and be in those moments reinforcing how you will go through those interludes in your actual performance. Be in the emotional performance of the song as well. Connect to the words as you “sing” them in your mind. Marry this emotional experience to the technical reminders you are sending yourself at the same time.
Mental practice of singing in this way strengthens the neural pathways responsible for giving the body the right directions to ensure a successful performance without actually having to sing to do so.