IPhone 6s battery usage for restart
One user's experience can differ from another for a range of factors (e.g. the age of your iPhone 6s battery, the number of charge cycles it's had, etc), so what is normal for you may not be normal for someone else.

An iPhone will typically use some percentage of battery over night, but how much will depend on whether it's fully shut down or just in Airplane Mode (your question doesn't really specify). If it's only in Airplane Mode, then the iPhone is not actually switched off and a number of sensors etc are still fully operational and therefore require some power.

Also, it's not clear from your question what you mean by several hours, but I think a 10% discharge over 6-8hrs is not uncommon.

You can find a lot more info by reading what Apple has to say about it's batteries.