The first 20 minutes of a nap is a light sleep and the second 20min is a deep sleep, with a partial awakening during the transition between the two. This is one sleep cycle. If a baby has not yet learned to get through this transition on his own, or if he is overtired, a wake-up is possible at this point.
The best thing to do when initially teaching sleep is to help your baby through these awakenings (often babies experience ‘jolts’ at this point).
There are a couple of ways of dealing with this, you can choose either, or both, depending on what you feel is best for you and your lo.
-Most important is a good swaddle. This will help him get through his jolt by catching his arms and legs and preventing them from flailing about, waking him up and scaring him.
-Secondly, after he goes to sleep, you can stay with him and help him through this jolt - keep a firm, gentle pressure on his back/tummy after he goes to sleep, and after about 20 mins you will feel the jolt. The pressure and the swaddle may be enough to help him through, or you may need to do a bit of 'ssshing' to sooth him back to sleep. If you do this for a few days (don't worry if it doesn't work the first time, a bit of persistence will pay off here), he should learn to go through this sleep transition on her own, and you won't have to do it again.