Morning light and cortisol - what happens when the day begins in darkness?

For much of the year, mornings in the Nordic countries begin in semi-darkness. It's dark when we wake up, dark when we go to work and sometimes it stays quite dark well into the morning. For the body, this is not just a matter of mood, but also a biological signal that helps the body set its circadian rhythm and

the timing of hormones.

With this in mind, it is particularly interesting that a new clinical study has investigated how light levels during the morning hours affect the body's internal rhythm, including cortisol, the hormone that plays a central role in alertness, stress

response and sleep/wake balance.

How did the study go?

The study involved healthy, young adults. Over five days, the participants spent their mornings, between 8 a.m. and noon, in different lighting environments.

One group was exposed to very low lighting (around 55 lux – roughly like dim indoor lighting), while another group was exposed to significantly brighter morning light (around 800 lux). The researchers then monitored changes in hormone patterns, sleep structure, and the participants' perceived alertness and mood.

What did the researchers observe?

One of the clearest findings involved cortisol, a hormone that is closely linked to the body's circadian rhythm. Cortisol is normally higher in the morning to help the body wake up and mobilize energy, and then gradually decreases throughout the day.

In the participants who spent their mornings in very dim light, the researchers saw that this rhythm changed. The group had:

• elevated cortisol levels later in the afternoon and evening

• approximately 25 minutes shorter total sleep time

• a shift of deep sleep to later parts of the night

• increased daytime fatigue and more depressed mood

  
  

The researchers described this as the body effectively entering a state of “biological darkness,” even though it was daytime according to the clock. In contrast, the group exposed to stronger morning light saw:

• more stable cortisol patterns

• a more balanced sleep architecture

• better alertness and more even mood during the day

  
  

The difference between the groups was not in evening routines or nightlights, but in how the day began.

Cortisol, light and women in hormonal change

Cortisol interacts with several other hormones in the body. When hormone balance changes, such as during menopause, this interaction can become more sensitive. Many women describe changes in stress response, sleep, and recovery during this phase of life.

In this context, circadian rhythm signals – especially light early in the day – become particularly interesting. The study contributes to the knowledge of how our light environment can affect the timing of cortisol, and how this in turn is related to sleep and perceived energy across the day.

Why is this particularly relevant during Nordic winters?

For much of the year, mornings in the Nordic region begin in darkness. The natural light signal that helps the body set its circadian rhythm can therefore be weak or delayed. The study provides a perspective on why dark mornings can be experienced as extra demanding – not only mentally, but also biologically – through how light, or the lack of it, interacts with our endocrine system and can affect hormones such as cortisol.

At the same time, it may become even clearer why the morning light feels so longed for when spring returns. After months of dark mornings, the body once again receives stronger signals that the day has begun. Getting outside for a while early in the day, even in normal daylight, can be an easy way to help the body find its rhythm again.

In short

• Morning light serves as an important starting signal for the body's circadian rhythm

• The study showed that dim morning light was linked to altered cortisol rhythm, sleep and well-being

• Stronger light early in the day was linked to more stable hormone patterns in the study

• The results provide context for how light, hormones and everyday life interact, especially in darker environments.

If you're interested in reading the full paper: 🔗https://pubmed.ncbi.nlm.nih.gov/41240877/

We enjoy sharing research that helps put everyday things into a larger context.

Go take that morning walk!