If you use your voice for your profession; i.e. you're a teacher, salesperson, work on the phone etc., pay particular attention to how you feel physically.
When you're fatigued, make sure that you are breathing consciously and filling up with enough air to support your voice.
You'll also find that attention to your breath, and using low, slow and full breaths
will not only powerfully support your voice, but will energize you in the process!