Sometimes you simply need a little rest. You may not always know why, but your mind and body will know.
Some of the more common reasons you’ll need a rest are: reduced sleep over the last 1-5 days, recent physical exertion, minor cold or flu coming on, minor infection somewhere, exhaustion (from long hours working (mental or physical), relief (from stress or after pushing your limits for awhile), ageing and occasionally it comes down to simple dehydration.
Whatever the reason, listen to your body. Have a rest. If the need for rest persists for several days and you can’t attribute it to anything specific, check in with your doctor. You could be low in iron or have something more challenging that needs medical attention.