Recently, I just started meditating and one of the skills I have been practicing is focusing on my breathing. When I try to do this for 5 min, I start to doze off towards the end, even when I am sitting upright. Is this normal? Also, I have been practicing before going to bed every night, so dozing off is not quite a problem. But if I need to calm down and mediate a little during work, dozing off is going to be a problem.... how can I overcome this?