You probably need to spend a little time investigating because this isn't a simple "check the aerial plug2 sort of problem.
Where I would start is by checking the signal late at night direct in to the TV. That means taking the aerial lead out of the PVRs and plugging it directly in to the TV, and making sure that the TV is correctly tuned. If you still lose signal late at night, then you've got an issue with the aerial system or TV, so you know you shouldn't need to waste time looking at the PVR box settings.
For a TV to cause an issue, it would have to put power up the coax to knock out a remote powered aerial amplifier. I've come across this with some LG TVs in a couple of rare instances. Either the customer had gone through some menus and decided that turning any "helpful" features on would be a good idea, or the TV was changed and the default setting for Antenna Power was ON but no one checked or understood to check whether it was needed. There may also be the case of the TV firmware being updated and some settings returned to default rather than how a customer set them.
Obviously with the above, it only really applies if you have an aerial amp, and it's hard to see how this would only affect things during a specific time, but it's still worth checking.
While in the TV menus, check for any timer settings or timer recording settings. Again, this is just eliminating possibilities that are easy to tick off. Where the signal still goes off, and there's nothing from the above that's applicable, then there's a good chance that your system is picking up some local source of interference.
You haven't gone in to detail yet about your aerial system; its age, condition etc so the following is simply the next step in a sight-unseen troubleshooting list.
Digital TV is more prone to interference affecting the signal. Poorly-shielded coax such as old TV aerial leads or those inexpensive aerial fly leads with the moulded-on ends are a common cause. Maybe the aerial is past its best, and while picking up enough signal still to give a useable result, when the interference strikes it tips the balance just over the edge in to failure. Another possibility is that the system is over-amplified and the interference is again the straw that breaks the camel's back.
Check through any connections and make sure that the aerial plugs are still fitted well enough to get a decent signal connection. Single-shielded coax can be a weak spot, so aerial systems with old coax should have the cable upgraded to double-shielded for digital. Use good quality all copper coax: Webro WF100, Labgear PF100, Nexans copper NX100. Replace any cheap moulded-end fly leads with ones made up from the spare coax. Look at the aerial wall plates if fitted. If you see just a simple 1" square circuit board with screw clamp terminals then it's probably unshielded too. Replace that with a shielded wall plate.
Check this stuff out and come back to us with more info.
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