I've recently (re)started my Zazen practice but I seem to be encountering a lot of 'tiredness issues'. I don't sit for long, but soon after I start I feel I need to struggle to remain away and my locus of attention moves from my breathing to tensing my legs in en effort not to fall asleep (at least, that's what it feels like). Note I sit Zazen in a lit room with eyes wide ope, and it really doesn't matter what time of day I do my sitting.
I'm not sure what to do about this - sometimes I feel it is better to stop than force it. Is this common for beginners, or is this just a very clear signal coming from my body that I need more rest?