It’s one of those phrases people have either a) heard or b) said themselves: “Is it cold in here, or am I just tired?”

On the face of it, the two things might not seem to share any obvious common ground, but it’s a phenomenon many of us have experienced: when feeling tired or lethargic, the need to wrap up in layers and curl up under a blanket can hit you out of nowhere. 

But why does this happen? And how are sleep and temperature related? We asked Samantha Briscoe, lead clinical physiologist at HCA Healthcare’s London Bridge Sleep Centre, to tell us more.

Circadian rhythm

The main reason why feeling sleepy can affect how warm or cold we feel has a lot to do with our circadian rhythm – aka our body’s internal clock. As you probably already know, the circadian rhythm is responsible for a whole host of bodily functions, including the all-important sleep-wake cycle.

“The circadian rhythm is generated in a region of our brains called the hypothalamus and is affected by external cues such as the light-dark cycle of the sun, exercise and temperature,” Briscoe says.

Your circadian rhythm also has the power to influence your internal temperature – and it’s this ability which leads us to feel cold or shivery when we’re sleepy or tired. 

Why? It’s all to do with the way our body temperature fluctuates to prepare for sleep. “Our core body temperature is approximately 37°C but fluctuates by about two degrees throughout the course of the day,” Briscoe explains. “A drop in core body temperature is a signal for our bodies to prepare for sleep and body temperature starts to fall about two hours before habitual sleep time.” 

She continues: “During sleep, our core body temperature continues to fall, reaching a nadir in the early hours of the morning before rising steadily again throughout the day.”

In this way, while you might not be approaching your ‘habitual’ bedtime when you’re feeling sleepy during the day, the drop in body temperature you experience is a result of your body’s need for sleep – even though you’re not ready for it, your body’s internal clock is telling you it’s time.

Poor sleep

Poor sleep can also lead to feeling cold due to the impact it has on our activity levels, Briscoe adds. 

“When we are tired our energy levels are low and we likely move less, becoming more sedentary,” she says. “As a result, our metabolism and blood circulation can slow down – both of which can lead to us feeling colder than usual.” 

It’s clear that our body temperature can influence how well we sleep, and vice versa. But how can we ensure we’re helping our body to decrease and increase body temperature at the right times?

Briscoe explains that the main thing we can all do is try our best to stay cool at night, even though your body temperature drop may leave you feeling a little chilly.

“The cooler body temperature we experience when we are ready for sleep is not a reason to bundle up with lots of blankets; several studies have found that we sleep better when we are cooler,” she explains.

“Increases in core body temperature during sleep promote wakefulness and lower the amount of deep sleep we get. Studies have also linked insomnia to higher body temperatures.”

If you struggle with feeling cold in bed, Briscoe recommends trying temporary measures over a heavier duvet or thick pyjamas, such as “an extra blanket that can bed shed or a warm hot water bottle”.

Alternatively, when it’s too hot to sleep, taking measures to ensure your body temperature stays cool – such as using a fan, iced hot water bottle or light pyjamas – can help you to achieve deeper sleep. 

Images: Getty