We are had exactly the same problem with our 4 year old DS. I bought a dimmer plug that a big bright bedside light plugs into. We start off pretty bright at bedtime, then we we go to bed I turn it down so it doesn't disturb him during the light. It has banished the monsters in his room A bit like this
Gro clock here too, has variable light settings, ours is on dimmest and half under the bed and still lights the area round the bed, so can be much brighter when on full. Is blue light. You don't have to use it as a sunrise clock if you don't want to but handy if you do ever want to.