In autumn and winter, 12 hours of darkness and then some. If the headtorch fails, it's game over.
A backup light has to work, and ideally it should weigh.. nothing. For the primary light, there are hundreds of models to choose from. Many runners like the Lumonite Compass or the Mila Vega.
But what about the backup light, the one that has to be there but most likely never will be used?
Frustrating that it should be so difficult to do the simple thing. Nitecore has managed to get all the critical features into a small NU25 package that weighs 50 grams including a headband with seven hours of light for running! I'm using the NU25 2017 model - it has been revised with a USB-C charging plug.
Rechargeable with USB and a powerbank if needed. Four white light modes - low, medium, high and turbo. Two red light modes. Rain and waterproof 1 meter. Lockable so the light won't turn on accidentally. Battery level indicator.
In other words, everything you could wish for. In maximum Turbo mode it pushes out 360 lumens with a battery life of 80 minutes. Medium gives 38 lumens and is great for trail running for up to 7 hours.
Note: I have no affiliation of relationship with Nitecore