The beauty of Yoga is that it is not black or white. Rather, it is a tool that helps us to find the gray area, that unique place where we are in this moment, and to determine what we need NOW. When we are tired, stiff, or pressed for time it is not our physical body preventing us from practice. It is our own mind. We figure that if we can't do our full practice then it isn't worth doing anything. We don't realize that "progress" in Yoga is not measured in time spent on the mat, nor is it dependent upon having a practice worthy of the Yoga Journal's photo spreads. It is to give us the strength and clarity to still our crazy minds. Five minutes of yoga for that purpose alone is more powerful than hours of asana (poses) without intention.
There have been times where I've had to cut a practice short due to responsibilities. There have also been times where I was aching, sick, tired, and unfocused but practiced anyway. I might not have had practices that looked stunning on those days, but I know this: I emerged from my mat a thousand times more centered and emotionally stable then before I started. I have NEVER regretted a practice. One of the most amazing practices I've ever had was during a brutal ribcage injury (thanks to surfing). The pain forced me into mindfulness. My mind wasn't concerned about grabbing my legs in back-bending, it just wanted some peace.The result was feeling lighter than ever before. That day cemented for me the importance and the purpose of practice. Our practice, just like life, will rarely be perfect. As soon as it is we will find new challenges to conquer. Whether 15 minutes of breathing or a full 2 hour asana practice, the result is always the same.......and always crucial to living a better life.
Yoga Sutra I:II Yogas Citta Vritti Nirodhah; Yoga is the control of the fluctuations of the mind.