an adaptive challenge

I have a technical problem. Luca is no longer sleeping through the night, so neither is anyone else in the house. It typically starts at 2 or 3 a.m. when he tries to climb into our bed. We return him to his bed up to 10 times, with decreasing levels of patience, and it eventually culminates in a full-blown tantrum. Last night, for dramatic effect, he climbed into Andre's crib and woke him up so he could cry along with him. The lovely pattern has been lasting between 1 and 3 hours per night for the past 6 nights. I often end up so frustrated that I cannot return to sleep.

Addressing the technical problem at face value has failed miserably. Specifically, our approach has been to kindly but firmly return him to his bed every time. We have now resorted to locking our door, which led him to try harder by waking up his brother, screaming for milk, taking off his diaper, turning on the hall lights... I will spare you the rest. I'll just say that I'm drinking more than 4 cups of coffee at the office just to stay alert.

We have an adaptive challenge- changing our behavior and his surroundings so we prevent the issue rather than react to it. At the moment, I'm wondering what could be causing him to want to sleep in our bed. What could possibly motivate a well-behaved toddler to defy us so insistently, to suddenly stop sleeping soundly through the night? An analysis of recent events, such as routine, relationships, health, diet, has not led us anywhere.

I am wondering if he has developed a fear of the dark. Perhaps if I give him his Buzz Light Year doll to "protect" him during the night he may feel better? Have you had this problem and how did you address it? I could really use some help.