The times your really too tired i would just skip it and do it when your not quite as tired you could just do some short destressing meditaions if your too tired also and some are not quite as deep so its easy to stay awake i tend to do some healing meditaions when i am really tired. a good one to do is one were you put the back of the left hand flat on your lap and count the beats of you pulse with your right hand use your 3 middle fingers count 4 beats whilst breathing in and hold for your breath for 8 beats then count 4 beats out and 4 beats in again and hold for 8 beats do this for around 5 mins its great as you won't fall asleep and i do it sitting in a chair. If you have some you do already that are similar you could try them.