I'm the mother of 7 months old twins, and our babies are awful nappers, leading to irritability during the day. We have no idea if they are overtired, undertired, or bothered by other things. The schedule is as follows:
8-11pm - 8am night sleep. No problems once asleep, but can be very difficult to put to sleep.
8am wake - very happy and appears well rested 9:30am-10am - nap time. Naps are always anywhere between half hour to an hour. They usually wake up in a good mood.
12:30-1pm - nap. Most of the time, they wake up very cranky. Sometimes, they can be rocked back to sleep for longer. Sometimes, they can't. Usually, once put back to sleep, they will sleep for ~20 minutes before waking up crying again. This repeats until they refuse to go back to sleep.
4-5pm - nap. This one is usually a little longer, but still needs patting/shushing/rocking in the middle. They usually wake up looking still sleepyish but not cranky.
5pm-8pm - very happy during this time. Bath at 7pm.
At night, they insist on being nursed to sleep (cosleep). Nothing else will do. However, having twins, I can't nurse both to sleep at the same time. So one gets nursed, while the other is rocked by my husband. That baby is almost always screaming the entire time. He brings the baby over once I am able to unlatch and the first baby stays asleep. However, sometimes, the baby wakes up after being unlatched, so we're essentially starting all over again. I nurse the other one, while my husband rocks a screaming baby. Typically, the baby wakes up all smiley and happy, but gets cranky fairly soon afterwards. This is why the variable night sleep time. Trying to put them to sleep earlier is worse, as they wake up after about an hour and in a super happy/playful mood. Then they end up sleeping even later.
During the day, after they wake up cranky from the second nap, usually after a while of holding and showing them things, they are in a good mood again and play for a while happily. When we try to push that first wake time later, they get very cranky and unhappy. Usually, around 9am is when they start yawning and rubbing their eyes.
So, does this sound like undertired or overtired babies?