I'll be driving to Norway in November and intend to sleep in my car on a few nights. For myself I have a sleeping bag and a few blankets, so the temperature should be no problem for me, but I will have some electronic devices (Notebook, Smartphone and camera) with me and the cold temperatures are not good for the battery. How can I keep them warm over night?
There are three primary things to think about when keeping your personal electronic devices in a cold car (with you) overnight.
If you want to be able to use your phone and camera immediately in the morning (or at night), then they need to be relatively warm. As @Bee notes in her answer, for an iPhone this means above freezing. I personally run into this problem when using my phone as an alarm clock in the winter.
If you won't need your devices on short notice and can easily re-warm them (by putting them next to your body or turning on your car heater), its perfectly OK to let them get cold. In fact, since most consumer electronic batteries these days are lithium-ion, it may very slightly reduce the self-discharge rate to keep them somewhere quite cool. That being said, the primary concern is....
When you sleep in your car in cold weather, the moisture from your breath will condense on any cold surface, including your electronics. It goes without saying that electronics do not like moisture, and you should do your best to prevent this. One person breathes out a shocking amount of water vapor throughout one night, and I have personally damaged my phone by letting it sit out unprotected from the moisture.
Before going to sleep, put your electronics in a Ziplock (or other resealable plastic bag). Do this outdoors in the cold air, so that you don't trap warm moist air from your (presumably initially heated) car. Empty as much air as possible from the bag before sealing so that you can't accidentally "pop" it open.
If you need to keep your phone warm, put it flat against your chest or at your feet. I usually car camp with bib/overall-type snow-pants with a chest pocket, which makes it hard to damage a flat phone when sleeping. Chests tend to be soft, and there's no elbows or hips nearby to crush a screen. At your feet means you might kick it, but won't be able to put your full body weight on the screen. If you're worried about damage, wrap it in an extra blanket/shirt/jacket.
For the (electronic?) notebook and other large items, I wouldn't worry about putting keeping them warm during the night--if you will need them accessible and working in the morning, just wake up a few minutes early and put them in your sleeping bag while you're awake.
Just did a quick search online for the recommended temperatures for electronics and phone one for i-phones (as we all know, they are probably the most delicate so I'm going to assume this works, there is actually an article inside that to that effect):
Keeping your smartphone within its operating temperature
All smartphone manufacturers recommend certain acceptable operating and ambient temperatures for use of their devices. These vary from manufacturer to manufacturer.
Some recommendations by Apple for iOS devices:
Use between 0°C and 35°C (32°F to 95°F). Lower or higher temperature may change the behaviour of the device.
Store between -20°C and 45°C (-4°F to 113°F).
I very much doubt your car will get below -20 degrees so as long as you let your devices warm up again before using, sounds like you'll be safe!
At the end of the day, as @David mentioned, if it's just a few objects, keep them under the covers with you.
If the devices are fully turned off, sensible amounts of insulation won't keep them warm overnight as they won't give off any heat. If they're turned on and giving off heat, too much insulation can lead them to overheat. The power consumption can be rather unpredictable, for example a mobile phone that just about gets a weak signal will use more power running its radio than one with a strong signal or one with none at all.
It's not usually clear from specifications whether the "storage" temperature requires unusual forms of powering down, but lithium batteries are reasonably tolerant of the cold if they're not delivering much current.
The best way to ensure that devices are comfortable is to keep them at the same temperature as yourself, and keep yourself comfortable. But most electronics don't like having a human on top and they don't make good mattresses. Some form of window insulation is probably a good idea; it can make a huge difference to your comfort in the morning, and combined with your body heat will keep the car a little warmer.