Help... I left my interior lights on!

First the good news, you did no damage to the traction battery, it is completely disconnected when the car is not READY.

Once you start the car, it will begin recharging the 12 volt battery, which has been powering the inside lights.

 

You have basically reduced the battery strength and the gas engine is recharging it. Your mileage will reduce because of this, but as you state, the bars return to 8-9, so the engne has done it's job. Question: Have you returned the overhead light function to power off? No, i don't think you screwed up your traction battery......you merely restored it.

 

Left mine on the first night after buying it. Didn't harm anything. Your Prius has a 12v AGM battery that can be drained much deeper than a typical Lead Acid. Don't even worry about it.

 

If you use led interior lights, the power drain will be about 5 times less...
They are dirt cheap, and brighter than the lightbulbs and should never need to be replaced.

 

Your 12 Volt battery has taken a hit. You were lucky: an interior light left on overnight can kill a battery, unfortunately. Or put it near-dead. Don't just rely on assurances, see for yourself:

Get a half-decent ($30~40 range) digital multimeter, and check the voltage to get a more complete picture of what your battery's health is. If it's 100% you'll get a reading around 12.7 volts or higher. Under 12.6 and it's likely taken some damage.

You can do the voltage check under the hood, at the jumper location. It's in the underhood fuse box, check in the manual. Red lead to the jumper terminal, then black lead to a bare metal bolt or bracket on the car body or engine.

An "intelligent" charger is worthwhile, for events like this, and just for periodic maintenance. My pref. is CTEK 3300, but it's kind of pricey, there are lots of choices. It will likely take an overnight charging session. The CTEK has indicator lights to indicate when it's still charging (amber), and when it's satisfied the battery is fine and fully charged (green).

Another item to consider is a portable jump starter, for emergency use with dead battery.

The hybrids tend to have diminutive 12 volt batteries, and an inadvertant light left on can kill them. Cheap insurance is to turn completely off as many of the "convenience" lights as you can, if you're ok with that.