Gut health, fasting and sleep: what do we know?

Gut health and sleep are closely linked. Whether a period of fasting affects your sleep isn't entirely clear yet. Research mainly looks at indirect links via the microbiome. These changes can then indirectly influence sleep. However, their impact depends heavily on the context, how you follow the programme, and especially on what happens afterwards.

What does fasting do to your microbiome?

A 5 Day Fast programme is based on controlled calorie restriction and a temporary change in your food intake.

Because of this, the environment in your gut changes too.

In practice, this means that what bacteria in the gut “eat” changes. When their environment shifts, the balance between different bacteria can also alter, and with it, the microbiome itself.

💡 The microbiome is the collection of bacteria and other microorganisms living in your gut that contribute to good gut function.

Some bacteria associated with inflammatory responses may decrease, while others that produce substances supporting the gut might become more prevalent.

These shifts have been observed across various models of energy restriction (1).

Gut health and sleep: what's the link?

The connection between your gut and sleep is more direct than many people think.

First off, the gut and brain are physically connected via the vagus nerve, a communication highway through which both constantly swap signals. That's one of the reasons why the state of your gut can affect your mood, stress levels, and sleep.

On top of that, gut bacteria produce substances that can influence processes involving chemical messengers like serotonin, which plays a role in mood, stress, and sleep.

The gut also plays a major role in inflammatory processes. When the gut gets out of balance, certain substances can pass into the bloodstream more easily, triggering the immune system and increasing inflammation. Higher inflammation levels are often linked to lighter, more fragmented sleep, and poorer recovery.

💡 The 5 Day Fast programme includes seven supplements, including omega 3. Their presence can help support a more stable gut environment, which may indirectly promote better sleep quality.

Can 5 Day Fast improve sleep quality?

The answer is nuanced.

During the programme itself, your sleep might actually get disrupted. This does happen for some people. Calorie restriction creates physiological stress and heightens the body's alertness, partly through hormones like cortisol. Because of this, nodding off can be trickier and your sleep might feel lighter.

At the same time, other adaptations could have quite a positive effect.

Eating less, especially later in the evening, means less work for your digestion. Some participants find they sleep more peacefully as a result, while others don't notice much difference.

5 Day Fast doesn't guarantee better sleep. Results vary from person to person.

What do we know about fasting and gut health?

A dietary approach like 5 Day Fast can be interesting if you have poorer dietary habits, such as a high intake of ultra-processed food or sugar.

In that case, the programme acts as a bit of a reset, temporarily cutting down on these foods and allowing the gut to function in a simpler environment.

It can also be interesting if you already eat relatively well. In that scenario, the programme doesn't fix major imbalances, but it can help to further support an already stable gut environment.

The period after the programme matters too. That's when potential benefits can be reinforced, provided you gradually reintroduce fibre-rich and minimally processed foods.

What does this mean for you?

Research suggests that a period of fasting can temporarily shift the composition of your microbiome. What that means for you differs per person. Some bacteria associated with inflammation may decrease, while others that contribute to a healthy gut might become more abundant.

That balance can then indirectly influence your sleep. A more stable gut environment can create conditions that are more favourable for sleep, like less inflammation, less strain on your digestion, and better hormonal regulation, including serotonin.

During 5 Day Fast, your entire digestive system gets more rest. Because of this, some participants find they sleep more peacefully. When your stomach and intestines don't have to 'work' late into the night, your body can focus on its normal nighttime processes while you sleep.

What makes the difference:

• what you ‘take away’ from 5 Day Fast into the period following the programme

• your ability to stick to healthy habits

• your overall stress levels

5 Day Fast can be an interesting way to reset your eating habits and lifestyle, or to give your already good health a bit of extra support. On the other hand, it's less suitable if you have active gut issues or severe fatigue.