Actually, the US uses more power during the day in the summer - there is a dropoff in the night for both summer and winter. Night time use is somewhat similar. [1]
Cooling takes more energy than warming, so the summer daytime use is higher. Summer = warm evenings. I'm from Indiana - it was almost always cooler at 10am than 7pm, even in the winter. It takes time to heat up or cool down. I'll also mention that nights and weekends use less power because business and industry tend to shut down during these times.
Which would somewhat logically mean that despite the efficiency being worse during winter, it isn't as much of a strain because power demands are less.