There is a considerable lack of sunlight in winter, which impacts the internal body clock, affecting sleep cycles. Also, melatonin levels are high in winter; consequently, the body feels tired and drained, that's why we sleep more during cold weather.
Lack of exercise makes us feel sluggish, lazy, and even obese, since no calories get burned. This leads to increased sleep during winter. Doctors suggest taking a brisk walk (even within the house will do) to increase body movement.
Heaters certainly warm up the room, however, the air in the room tends to get even more dry. Excess (and artificial) warm air dries out the internal mucus membranes, increasing vulnerability to bacteria, and consequently disturbed sleep patterns.
The hormone 'leptin' is associated with metabolism. Though digestion is better during winter, excess of fatty foods cause a change in the leptin levels of the body, leading to disturbed sleep. To get good sleep, avoid a very heavy dinner.
Winters cause a change in the circadian rhythm, leading to fluctuating sleep patterns, and vice-versa. Too much or too little sleep affects the body. To avoid mental and physical exertion, avoid the tendency to oversleep, and try and regulate sleep patterns.
Maintaining the right room temperature, between too hot and too cold, helps us sleep better in winter. Ideal room temperatures vary from person to person, though experts state that anything between 60 to 70°F is ideal.
Cramps, sore throats, etc., are no surprise. This creates a vicious circle - sickness prevents us from sleeping peacefully, but a sound sleep is necessary for the sickness to disappear. To sleep better, avoid physical exertion as much as possible.