As I said in chat naps only really work around 5-7 hours after waking up for most people like sisyphus said. Generally it is caused becuase we have exited our morning awakeness but have not yet entered our evening awakening cycle, usually napping is done right after sleep and we are slightly exhausted because it is the hottest time of day. SInce we have no need for restorative sleep our body largely skips NREM and jumps straight to REM.