See he cannot force you or threaten for same you can use your light in your home freely.

If he illegally threaten or force.you, you can file police complaint for intimidation.

You are well within your right to light up your room. It is not your fault.  Tell them to make a shade of the light by any object. 

See they can install curtains further you can use some more cover to settle it amicable.

You may ask them to take any legal action on this because ther is no provision in law for taking any legal action on this.

However it becomes your duty also to not to disturb the neighbors for any reason.

You may ignore their complaints.

Let them go and complain about this anywhere, no complaint is maintainable.

 covering through curtain is more than enough no one can take objection for the same. They can also use curtains for the same

You are at liberty study at night and keep your lights on 

2) your neighbour has no business to object 

3)you cannot be forced to switch your lights 

Tell your neighbour to draw curtains in his house and sleep 

no need to bow down to pressure tactics 

Tell them, put curtain at your side too, No bar to use light at night that also for study purpose.

If a neighbor’s actions continuously interfere with your enjoyment of your property, you can sue to put an end to the behavior. 

Dear 

You need to take precautions that light doesn't disturb your neighbours.

If even after putting curtains over windows they are complaining then you should ask your landlord to talk to them about the issue because you need light for studying and not for partying.

They need to understand your necessity and they should also take some precautions that light doesn't disturb them. 

you can use light whole night. permission not required from your neighbour to use light.