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How Does Sleep Actually Affect Weight Loss?


In the previous post we looked at the effect that sleep deprivation has on our ability to lean out and drop unwanted body fat.

A lack of sleep or shorter sleep time has been strongly associated with a higher body mass index (BMI) and weight gain [1] [2].

T.S Wiley in Lights Out! suggests that this is because we have evolved over time to maximize the feast-time of summer in order to cope with a potential famine in winter.

Our ancestors lived in sync with the day/night cycle.  They traditionally slept less in the shorter nights of summer, and spent the longer days mating and eating calorie-dense carbohydrates (feast-time) to store as fat to deal with likely food shortages in the coming winter (famine-time) [3].

Regardless of whether or not this is the case, less sleep means less opportunity for hormones that are secreted at night time to reach their optimal levels.

This has a cascading effect that disrupts our bodies regulation of the rhythm of a whole heap of other hormones.

Essentially, even if we are doing everything else right like eating well and exercising sensibly, our hormones could be working against us to gain weight rather than lose it.

The Hormone of the Night

Melatonin is the “time-keeper” hormone that regulates the levels and timing of other hormones in our bodies.
It is a potent anti-oxidant made from serotonin and is produced in response to light levels, so it keeps our bodies in sync with the day/night rhythm [4].

Photo used with permission from Kathy Reid’s Red

When we see the warm ‘reddy orange’ light of sunset the signal is sent to the pineal gland to begin secretion of melatonin and we start to get drowsy.
Melatonin production continues over the night time and the ‘blue green’ light of day shuts off production in the morning when it is wake-up time.

However, inside lighting, computer screens and TV screens all register as blue green light in our visual field.
So the signal to begin melatonin production becomes delayed if we sit up to all hours in front of the TV or computer or doing other activities requiring light.

A wacky bedtime and shorter sleep time means that melatonin production is both reduced and out of sync with the day/night cycle.
This disrupts the timing and level of production of other hormones in our bodies including prolactin, leptin, estrogen, testosterone, cortisol and possibly insulin [5] [6].

How do these hormonal cycles affect our body fat levels?

The hormonal disruptions resulting from sleep deprivation can be lumped into two different camps in terms of their affect on our body fat levels.

A lack of sleep affects our bodies appetite control.
When we get less sleep we tend to crave highly rewarding, calorie-dense foods (think Epic Mealtime on YouTube) and we will often snack more (maybe because we have a longer period of awake-time outside of mealtimes) [8] [9].

A lack of sleep also affects our bodies energy mobilisation & storage.
Stress – either induced by sleep deprivation or contributing to sleep deprivation – causes energy to be mobilised from our muscle and liver tissues then re-stored in our fat cells.  Also incoming nutrients tend to be preferentially stored in fat cells and our bodies have the tendency to burn glucose for energy rather than fat [7] [10].

The Appetite Monster

Various studies have shown that shorter sleep is associated with increased levels of ghrelin and reduced levels of leptin both of which regulate our appetite control [12].

Ghrelin is a hormone that “tells us when we are hungry or low on energy” [7].
Increased levels of ghrelin will increase our appetite.
When we get less sleep our bodies produce more ghrelin.  [I don’t know how, I could find nothing in the common literature that explains how this happens.]

Leptin is a hormone that is produced both by fat cells and the cells that line the wall of the stomach.  It is responsible for reporting how much fat is in storage and for sending the “full” signal to the brain to tell us when to stop eating [7].
Melatonin increases our levels of leptin and our brains sensitivity to leptin during the night, so we don’t wake up and go off in search of food.
Shorter nights and less sleep mean that less melatonin is produced. So leptin levels are lower than they would normally be during the night but also the cascading effect of melatonin on the daytime production of prolactin reduces our leptin sensitivity during the day as well [3].

So, the end result of a lack of sleep is the double whammy of higher levels of ghrelin, which means you will want to eat more (usually those highly rewarding, tasty processed foods); and lower levels of leptin, which means that the food you do eat is not registered by the brain as satisfying or as “filling” as it should be (so you will want to eat more of it).

This is all well and good, if we are trying to store fat for the coming famine of winter.

But in the modern age, this survival mechanism becomes totally redundant in the face of an endless food supply and indoor living.  So it causes us problems with weight gain through excess carbohydrate consumption unless we can coax that sleep duckie back into line.

The Fat Cell Silo

Grain silos

Robb Wolf summarises this energy mobilisation & storage problem in The Paleo Solution.
Essentially, sleep deprivation is stressful on the body and reduces our ability to cope with other chronic stressors.  This will raise our cortisol levels both morning and night.
Cortisol is a hormone that has many functions, one of which is to help regulate our energy levels.
So, cortisol is naturally higher in the morning when we need to get up and go and low at night when we should be winding down and going to sleep.

Chronically high cortisol levels will unnecessarily mobilise glucose and fatty acids stored in the liver and throw them into our bloodstream.
But since there is no saber-toothed tiger to run from, that extra energy is not needed and especially not at night time.  This means it will have to be put back into storage so our bodies can keep things on an even keel.

Insulin has many functions but its main one is nutrient partitioning.
Insulin facilitates the uptake of blood glucose, fatty acids and amino acids into our muscle, fat and liver cells, it decides what will be stored and where.

However, in this situation there is only so much insulin can do to convince the liver and muscles to take up the excess glucose and fatty acids (especially as high cortisol levels will interfere with this), so the only other place to shunt it off to is our fat cells which continue to be insulin-sensitive long after other tissues have become resistant to the action of insulin.

The fat cells around our bellies are the favourite place for fat storage in this situation – think about the high powered and undoubtably highly stressed business man with the pot belly.

The Subsidised Mineowner

Consistently high cortisol and insulin levels will lead to insulin resistance and leptin resistance, so our bodies will ignore the signals to stop eating then store the extra glucose we consume in our fat cells, or convert it to fatty acids (to store in our fat cells) or worse yet leave the glucose circulating in our bloodstream driving our blood sugar levels up and causing vascular damage.

…when insulin levels go up, we store fat.  When they come down, we mobilize the fat and use it for fuel.”
~ Why We Get Fat, Gary Taubes

If our insulin levels do not come down, or do not come down for long then we increase our opportunity to store fat and reduce our opportunity to burn it [10].
Our bodies will also preferentially use any excess glucose in the bloodstream as an energy source (to get it out of the bloodstream) over burning our fat stores for energy.

The Added Bonus

As an added bonus just to round it all out: cortisol and melatonin have an inverse relationship, when melatonin is low, cortisol is high [11].
So, when signals to increase melatonin production are delayed (like when we stay up watching TV or surfing the net), cortisol levels will stay high.
This will also push up our blood glucose and insulin levels when they should be winding down for bedtime.
Which makes it hard for us to get to sleep and stay asleep, putting further stress on our bodies and continuing the pattern of high cortisol levels [7].

It all becomes a very vicious cycle!

Take home messages:

  • We are not the ‘Sleepless’ of the genetically modifiable future so we cannot function without sleep, we are just not wired up that way.
  • Not getting enough sleep makes us stressed, reduces our ability to cope with stress, increases our appetite and reduces our fat-burning potential.
  • We tend to overeat carbs because that is what we crave in this state and they will generally be stored as fat.
  • If our sleep duckies step out of line and wander off they leave behind issues with appetite control and energy mobilisation & storage.  In order for us to clean up the mess that sleep duckie has to get back in the row!

So the advice is to:

sleep as much as you can without getting fired or divorced” ~ Robb Wolf

though Robb Wolf does tack an extra caveat onto this:

…if either the job sucks or the marriage is dodgy, then maybe those are worth shelving”

There is plenty of advice about for how to get a better night’s sleep.  Check out Marks Daily Apple and Chris Kresser’s Healthy Skeptic Blog.
Both sleep time and sleep quality are important.
In the next post I will share some of the things Andrew and I have done to improve our bedtime and quality of sleep.
You can also check out the Eat. Sleep. Move Facebook page for more regular updates.

So, until next time: eat, sleep move…then get some more sleep, it couldn’t hurt!


References and Further Reading, Watching or Listening:



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