In case of hibernation it seems very plausible that an animal evolved to sleep longer and longer, to preserve energy. I don't understand your problem? It could

In case of hibernation it seems very plausible that an animal evolved to sleep longer and longer, to preserve energy. I don't understand your problem? It could also work this way: an environment had enough food to sustain 20 bears in winter. If they sleep longer, the environment can sustain 40 bears. If they sleep even more, 100 bears can survive. The population would then grow from year to year, while sleeping more and more. What books on evolution have you read?