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You may no longer be a believer but curling up in your bed on the cold winter night of the 24th of December, waiting for that jolly red man to arrive, anticipating what the morning will bring, is still the highlight of the year. When you’re wrapped in your bed covers all cosy, mimicking a burrito, spare a thought for the tall white guy in the sky, who's flying solo to bring you joy and happiness. Whilst being sandwiched between the fluffy white clouds and the stars, he will be experiencing sub-zero conditions and chilling winds with only centuries old leather mittens and a red cotton coat to keep him warm. You can argue that being sat in that sleigh will be very similar to what Edmund Hillary or Ernest Shackleton had experienced climbing the Earth's highest peak and exploring the poles of our planet.
When taking on such an expedition the human body burns through 2 to 3 fold more calories than an average man! This is not only due to the physical exhaustion experienced but also the need to stay warm. The human body does this in many different ways; such as thermogenesis, which we are about to explore. Thermogenesis, literally meaning "heat-making", is the main method by which the body maintains its optimal temperature of 37 degrees Celsius. However, heat means energy, therefore thermogenesis is also a huge calorie sink. Thermogenesis is when we convert chemical energy; in the form of adenosine triphosphate (ATP), into kinetic energy such as shivering, which we have all experienced from time to time. 
ATP is the product of aerobic respiration which occurs in multiple compartments of the cell and includes multiple different reaction cycles which we can study, one of which is glycolysis. Glycolysis is the cellular mechanism in which we metabolise glucose monosaccharides (6 carbon sugars) into smaller carbon chains via a mechanism of reduction and oxidation reactions. By doing so we produce a 3 carbon molecule known as pyruvate (2 pyruvate molecules per 1 glucose). Pyruvate can then enter the citric acid cycle in the mitochondria where it is further oxidised. The products of these reaction mechanisms (such as NADH) is then used is used to enable the transfer of electrons in the mitochondrial inner plasma membrane. As the electrons are passed from complex to complex through the plasma membrane, the electron loses its energy value from an excited state to a more grounded state. This energy is used to pump hydrogen atoms against its concentration gradient and into the inner membrane space of the mitochondria. By allowing the hydrogen atoms to flow back into the mitochondria, down its concentration gradient, through a protein known as an ATPase, we can form ATP. 
But where does this all fit into shivering? Well to contract your muscles and to relax them rapidly as seen in shivering, you need a lot of ATP. Therefore you need a lot of glucose to make ATP. Glucose is found in many if not all food as you break down your carbohydrates into their individual units.
These carbohydrates can also be stored in as fatty deposits in adipose tissue before being used to generate ATP. There are two types of adipose tissue; white and brown. White adipose can be found in many places such as the liver and muscle and its glycogen stores are frequently recycled to form glucose for respiration to occur. However brown adipose has a slightly different function. Brown adipose, or brown fat, is found in the muscle (more so in very active mammals such as adventurers) whose purpose is to generate heat. A lot of energy is required for this and is known as non-shivering thermogenesis . Because of this, these adventurers need to eat a high fat, high sugar diet in order to maintain these brown adipose fat deposits.
This is the same for that white bearded man in his sleigh on the night before Christmas. Hauling those presents up and down your chimney is no easy feat, let alone repeating it million of times all around the world. So after you have had your glass of cream liqueur (or milk) whilst sitting with your loved ones around that open fire and have decided to call it a night, leave a plate of cookies for Santa. He needs it more than you think!