These environmental signals are integrated in the brain, probably by the suprachiasmatic nucleus, which regulates the amount of melatonin released from the pineal gland. Melatonin seems to be the major hormone involved in controlling seasonal changes in physiology and behavior. Internal factors also affect the rhythms of hibernation. For example, as the bodies of hibernating male squirrels return to normal in the spring, a sustained, increased secretion of sex hormones prevents a relapse into hibernation. Because the end to hibernation is so sensitive to temperature, increased atmospheric warming is having a noticeable effect.
Recent data show that the hibernation period for some animals is becoming shorter.
Yellow-bellied marmots living in the Colorado Rockies, for example, are emerging from hibernation almost five weeks earlier than usual. Snowfalls have also increased with the changing climate. The ground is still covered with snow when the marmots finish hibernating, making food hard to find. If these trends continue, the marmot may experience problems.