Why is my turtle sleeping so much

What temp is your basking area and water? Do you have a UVB light, a water heater and a heat bulb? My house is cooler in the winter than the summer. My water heater is set for 76. In the winter the water is usually between 74 - 76. In the summer my water temp is around 78. My basking area in the summer is 90 - 95 in the winter it's 86 - 88. I think a slight change in temps may cause them to bask more. Mine basks slightly more in the winter. I haven't seen him sleeping more though. I guess your turtle may be suffering from a seasonal disorder. Keep an eye on him though. If he is sick he'll hide his illness.

Stupid is as stupid does. - Forest Gump
The only dumb question is one not asked.