My boys never ever take monster naps like that unless they need it (ie: illness coming on, physical or developmental growth spurt, over stimulation), so I usually trust that their body needs it on the off chance that they actually sleep that long. However, now, even on days that they're under the weather or something, I cap it and go with an EBT because as they've gotten older (ie: almost 3 and newly 5) allowing a significant amount of additional day sleep gets us into an UT/OT cycle insanely fast. At 19 months though, I usually let them take the lead on those odd (and incredibly rare) long nap days.
