I'm assuming the main room light is mains-voltage (tungsten or CFL)? Is the bed-lamp too, or is it a low-voltage LED affair?
Here's the basis for a possible circuit which I think would do what you want, but its output would need an additional driver stage to suit the bed-lamp type :-
The LM324 is just an example opamp, used here as a comparator U1.
Photocell CdS, R1,R2 and R3 form a voltage divider string. R1,C1 develop the short-term (~100ms) average of the cell output to filter out lamp flicker effects. R5,C2 develop a longer-term (~1s) average of the cell output reduced slightly by the R2,R3 combo and hence provide a (variable) ambient illumination reference at the inverting input of U1. C1 voltage is normally just above C2 voltage.
If there is a sudden drop in illumination (e.g. when the room light goes off), C1 voltage drops relatively quickly and goes below C2 voltage so U1 output goes low, setting a latch formed by cross-coupled NAND gates U2a,U2b and providing a high output to turn the bed-light driver on. C1 output then rises again, but U2a output goes low, pulling down the C2 voltage via R7,D1.
Operation of the 'Off' button resets the latch, turning off the bed-light driver. The resulting C1 voltage drop cannot now go below the C2 voltage, so the latch does not get set by the bed-light switching off. With the latch reset, C2 voltage rises again slowly and the circuit is restored to its standby state. An 'On button allows the bed-lamp to be switched manually, independently of the auto switching.