It's not unnatural to become tired at times in meditation... afternoons are actually the most common time to experience this. However, there are a couple of possible reasons for the tiredness you are experiencing.
One, is that you are releasing some deeply rooted "stress" or "psychic/emotional toxins." This can sometimes bring about a tired feeling, in or out of meditation. If this is the case, the tiredness should pass when the release cycle is complete.
The other possibility is that you are, in fact, tired - either on a day to day level, or at a deeper level of your being - and need a bit of extra rest. If this is the case, I suggest you take a short nap at the end of your meditation (if time allows). Post-meditation naps can actually be very healing.
Whichever it is, experiencing tiredness when meditating does not mean there is anything wrong. It's all part of a gradual process of healing and transformation.
Me: 35|WAW: 38|D: 6yo | http://tinyurl.com/2dxx7m6 Feb 2006, left, came back in two weeks Aug 2006, left again Apr 2007, filed for divorce Dec 2007, reunited Mar 2010, moved out, filed again
