Don't practice when you are tired or continue when you begin to get tired, mentally or physically. Practice without awareness is futile and dangerous, because you will practice mistakes and poor posture when you can't pay 100% attention to what you are doing. It is better to take a break every ten minutes for at least five minutes than to practice longer, especially at the beginning. It may seem excessive to break so often, but your body and brain will thank you in the long run. It is very helpful to take a class in the Feldenkrais method or Alexander technique as they help you to become aware of your body's needs in terms of movement and posture. They also teach the most efficient way of using the body, as opposed to working against it.