We're three days away from leap five for our five month old and today he's either been sleeping or really unsettled, but tonight he'll barely go down to sleep before he's awake and upset again. I think his incoming teeth are bothering him so I've tried the usual tricks to ease the pain before bedtime but it's exhausting.
Your son could be going through the four month sleep regression which can happen any time between three and five months. Your baby is going through a stage of development when these occur and they can affect his sleep temporarily. The four month regression is the most prominent one and is when babies become more aware of their sleep cycles. Their sleep cycle is around 45 minutes and you may find they start to wake more frequently at night and have shorter naps. As they become more aware of their environment and will cry out for you more. We can support babies in linking their sleep cycles by putting to bed before they get overtired
To help with these transitions, you can try to put your baby to bed before he gets overtired. Look at his awake window, which for a four month old is around 1hr30-1hr45. So putting him in his bed before he gets to the end of his awake window can help him to self settle. You will get a rough idea of when your baby is getting tired with a few signs.