A drop in metabolic rate means a drop in heat production. Metabolism can drop as much as 10% in humans during sleep! Actually sleeping at night could also be the reason for the temperature drop. As night is colder than a day, keeping your body temperature higher would cost more energy.
"Always being cold is a telltale sign of hypothyroidism, which means your thyroid doesn't secrete enough thyroid hormone," Holly Phillips, MD, a board-certified internist with a private practice in New York City, told Health.
Lack of sleep. You might get the chills when you don't get enough sleep. Some research suggests poor sleep hygiene can cause you to feel cold more frequently. Sleep-deprived people may be more vulnerable to heat loss and unable to feel warm even at temperatures thought to be associated with comfort.
Sleep in soft, flexible fabrics. Flannel will keep you cozy, but it still breathes so that you won't get too hot, or damp from sweat. Silk could be warm, too, but it might not breathe as well. If you're really cold, long underwear and even a hat, or “nightcap,” can help.
Hypothermia is a medical emergency that occurs when your body loses heat faster than it can produce heat, causing a dangerously low body temperature. Normal body temperature is around 98.6 F (37 C).
Lack of vitamin B12 and iron deficiency can cause anemia and lead you to feel cold.
Whether it's a decreased circulation or something that's blocking your circulation, poor blood flow is another potential cause of your chilly disposition. “People with a decreased circulation to their extremities are definitely going to feel cold, especially in their hands and feet,” says Dr. Morgan.
Can low vitamin D cause you to feel cold? Instead, vitamin D deficiencies typically result in rickets and other bone deficiencies.  While these conditions can lead to feelings of coldness, they don't seem to play as direct a role in thermoregulation as the B vitamins.
Cold hands and feet
A vitamin B-12 deficiency can lead to the feeling of cold hands and feet, numbness, or tingling. Unfortunately, up to 15% of the general population is deficient in this important vitamin. To boost your intake of vitamin B12, consume more meat, poultry, eggs, milk, and other dairy products.
Often, this is our body's response to being cold. But you might not know that chills can also be a symptom of anxiety. Physiological reactions to anxiety can cause your blood to flow less efficiently and, therefore, leave you with chills. If you have chills from anxiety, you can begin to feel shaky and start to shiver.
If your blood pressure drops too low, your body's vital organs do not get enough oxygen and nutrients. When this happens, low blood pressure can lead to shock, which requires immediate medical attention. Signs of shock include cold and sweaty skin, rapid breathing, a blue skin tone, or a weak and rapid pulse.
You Don't Get Enough Vitamin B12
It can bring on anemia, which can make you cold. You can get vitamin B12 when you eat chicken, eggs, and fish. Some cereals and other foods are fortified with it, too. Keep in mind that you may not get enough B12 even if you eat lots of food with it.
Underlying arterial disease could be causing inadequate blood circulation, making it hard to maintain core body temperature, especially in your extremities. Smoking affects temperature regulation in a similar way by causing blood vessels to constrict.
Vitamin B12 Deficiency
Along with iron, vitamin B12 plays a crucial role in the production of red blood cells. Just the same, therefore, a deficiency may lead to persistent feelings of cold.
Hypothermia is a dangerous drop in body temperature below 35C (normal body temperature is around 37C). It's a medical emergency that needs to be treated in hospital.
Also under a circadian cycle is our core body temperature. We reach our minimum temperature about halfway through our sleep cycle. By the time we wake up, our bodies have warmed up slightly, but often not yet to our typical body temp. So, we wake up feeling cold because we are cold.
Overall, this is considered a normal temperature on each life stage: 0 – 10 years old: 95.9 F (35.5 C) to 99.5 F (37.5 C) 11 to 65 years old: 97.6 F (36.4 C) to 99.6 F (37.6 C) 65+ years old: 96.4 F (35.8 C) to 98.5 F (36.9 C)
Sleeping in a cold room is okay as long as you're comfortable with the temperature. If you're healthy, the crisp air is unlikely to make you sick. Instead, sleeping in a cold room has many benefits like reduced insomnia symptoms, better melatonin production, and deeper sleep.
Wearing clothes in a sleeping bag definitely keeps you warmer, so long as they're the right clothes, but they also help to regulate temperature and moisture levels.
In addition to shaking or trembling, other typical forms, according to calmclinic.com, include arm or leg spasms, cramping or longer or slower shaking than usual. Freezing hands and feet. Stress and anxiety can decrease your circulation. As a result, your hands and feet may feel icy.
•A consistent sense of feeling pressured and overwhelmed over a long period of time. •Symptoms include aches and pains, insomnia or weakness, less socialization, unfocused thinking. •Treatment includes lifestyle changes, medications, setting realistic goals.