My 4.5 month old takes crap naps at home. He wakes up every 20-30 minutes and cries. Sometimes I just let him keep crying in hopes he'll fall back asleep. Most of the time he gets so worked up, that's it. Even if he falls back asleep, he'll repeat the wake-up/cry thing every 20ish minutes. How can I get him to get past that REM cycle, if that's what it is? Is this a sleep-training issue?