Sleep goes in cycles at certain points we are all closer to waking than at others.
Do you and or DH tend either to be slightly quieter in the hope of not waking him?
I found my DDs tended, very irritatingly, to wake up just as we went to bed. I think, small children are so used to sleeping when everyone else is awake, silence frightens them. Our house is partly open plan and upstairs is never totally silent if people are watching TV, talking or moving about.
I have a DS 15 months old who started sleeping through very early at around 10 weeks old, and has done ever since (I know I'm lucky so please don't hate me).
Thing is, he always wakes up once after about an hours sleep every evening. He goes to bed at 7.15/30 and is asleep within 15 or so minutes after no distress at all (sometimes he chatters but he's very happy to be in his cot). Then at 8.30-9pm he wakes up crying every single night. I we leave him to cry he just gets more upset. So either menor DH go up to him, pick him up and resettle. Within 2 minutes he's back to sleep until 8am the next morning with no further wakings.
It seems like a habit but when we go up he also seems to still be asleep (but crying IYSWIM). Maybe nightmares (or is it too early for that?). Plus would nightmares happen at the same time each night without fail?
This doesn't happen a nap time (one perfect free noon nap for 2 hours after lunch without any wakings).
Any advice would be so appreciated - we just don't know what it could be...