Just that. DS is 4 months.
My son tends to yawn and start cuddling his teddy and stroking it against his face, so I know he's tired. I put him down in his cot with the mobile on and he can lay there for quite a while with his eyes open!
He's not upset, he's just enjoying his dummy and looking at the mobile. Occasionally he rolls his head from side to side which he does when he is sleepy.
But I feel bad. I feel like he's bored or I'm not stimulating him enough or I should take him out if he isn't sleeping straight away. He's been in there for about 15 minutes and he isn't asleep.
He's a very good baby but I'm an anxious first time mum.