Actually, this is common - sometimes because the cool and commonly calmer night air causes the smell to settle and concentrate in lower areas, which then dissipates as the air heats in the sunlight and starts moving around. This commonly happens in the case of dead animals in the walls or crawlspace/basement.
Also, if a live animal smell (look around foundation and crawlspace screening for any sign of entry holes, or of burrow holes which might go under the foundation and pop up in the crawlspace), and it is hanging out under your house in the nighttime and moving out in the daytime, smell would be expected to be stronger overnight.
Here are links to several previous similar questions with answers, including checklists of common sources of this sort of smell which you could check out:
https://answers.angieslist.com/Every-...
https://answers.angieslist.com/i-odor...
https://answers.angieslist.com/Musky-...
https://answers.angieslist.com/I-sewe...
https://answers.angieslist.com/Horrib...
One cause which I may not have included in those answers - most common in fall as the furnace starts kicking on only at night - if you have steam/hot water heating and it is just kicking on at night at this time, and is leaking a bit - could be wetting a dryrotted spot at a leak and causing it to smell, or if you have not been annually performing boiler maintenance and flushing some water out of the system, could be the stagnant boiler water itself (or sweetish smell if your system has antifreeze in it, as some do).
If unable to track down yourself, then I would bring in an Animal Removal (your Search the List category to find well-rated and reviewed vendor for this) expert to check for animals (dead or alive).
