On the Kudu and Zebra trail rooms (facing the arusha savanna) the sun rises out there and it is sunny all day - these rooms are very bright and cheery.
I would imagine the rooms across the hall on the other savannas and the ostrich and giraffe trails would have MUCH less light during the day with some direct sun in the late afternoon.
Perhaps those who notice the dimness most are guests not on the Arusha savanna.... honesty even in the dead of night the lights in the room were plenty bright for changing, reading, make up etc...
I have heard about people bringing higher watt lightbulbs with them - I think this is overkill.