I've experimented with my baby's sleep the last two days by not putting him in his cot for naps. My baby (14 weeks corrected, 20 weeks by birth) will sleep for hours on my chest a couple of times a day, hardly even waking to breastfeed before going back to sleep.
However, if I put him down in his cot he will usually only go for 45 minutes. This will need to be repeated often but the time it takes to get him to sleep means he doesn't get anywhere near as much as if he sleeps on me.
It isn't practical for me to let my baby sleep on me all day but there is such a big difference in quantity it must have an impact somehow?