Sleep, Sleep, Sleep! How artificial lighting and cortisol impact zzz’s
Thursday, February 23rd, 2012
Hey there, been getting a lot of questions about the recent BBC piece on sleep. If you have not read that yet please do and then skeedadle back here.
So, the thrust of the piece is that “normal” human sleep (both from a research perspective and anthropological observations) is NOT a solid, 8 hour block. Research and anthropological observation indicates people should experience alternating periods of waking and sleeping. The article makes the point that many people experience “sleep anxiety” when they awake at night and then fret that they will not get back to sleep.
Ok. This article seems to have really spun people up and it’s produced not much more than a shrug for me. Why? Because instead of thinking about all this as an isolated concept I’m thinking about it from the larger evolutionary biology context (plus factoring in what I know about sleep research). Let’s revisit the opening section of that BBC piece:
In the early 1990s, psychiatrist Thomas Wehr conducted an experiment in which a group of people were plunged into darkness for 14 hours every day for a month.
It took some time for their sleep to regulate but by the fourth week the subjects had settled into a very distinct sleeping pattern. They slept first for four hours, then woke for one or two hours before falling into a second four-hour sleep.
That “14 hours every day for a month” is my emphasis. Let’s think this through: If you live at the equator you generally experience about equal length of day and night (neglecting electricity for the moment). If you live at higher latitudes (either north or south) you will see greater variability in day/night lengths especially at very high latitudes in which folks may have 24 hour light (or dark) depending upon the time of year. Folks in these extreme areas tend to report a bit of…”squirrleyness” when the light/dark cycle is perturbed. Obviously folks adapt to and survive these extremes but it’s not comfortable relative to the periods when they have a more reasonable light dark period. What the researchers did in the highlighted piece above is restore the OPPORTUNITY for something approximating our normal wake/sleep cycle. Folks were allowed 14 hours of darkness. Not far off what we’d see at equatorial levels most of the year…and they experienced “normal” broken sleep patterns. The key points here should be obvious but I think folks are missing them:
1-These people had 14 hours to potentially sleep.
2-How often does this happen in the modern world?
Our modern sleep patterns of a 8-ish hour blocks is an attempt to deal with a chronic stressor. We do not have as much time as is genetically wired into us for sleep, so our bodies do the best they can. When you look at the sleep of people who have stayed awake for many days you see this pattern played out but to an even more extreme degree. People who have experienced very long term sleep deprivation tend to bypass REM sleep and dive right into the deep level 3-4 sleep which tends to be the most restorative. We all tend to do much the same thing, just to a lesser degree.
That’s one piece of the puzzle, just the “sleepy time” segment, but we have another piece to this which relates to total photo period and peaks and troughs in luminosity. The ancestral environment contained both more and less light exposure. We experienced more light from an intensity standpoint in that we got outside and had the sun on our skin (and in our eyes, establishing our circadian rhythmicity and setting the tone for the production of sleep hormones such as melatonin) and less in that once the sun went down we had starlight, moonlight or firelight. Indoor lighting by contrast is this kind of grey purgatory in that it is many orders of magnitude of intensity LESS than sunlight, yet many orders of magnitude of intensity GREATER than starlight. Indoor lighting is like chronic cardio relative to lifting weights and sprinting: not enough of an acute (hormetic) stimulus, too much of a chronic stress.
Check out this chart of relative luminosity:
It’s important to note that this is a logarithmic scale. The difference between direct sunlight>office light>starlight is not just “big” but “really big.” Why this is significant relates to a host of biochemical cascades tied to photoperiod, not the least of which is cortisol level. In this graphic we see a simplified cortisol level throughout a normal day:
Cortisol is relatively low at night (allowing us to sleep) higher in the day to stabilize energy levels and modulate immune function. Cortisol is highly responsive to photoperiod and “long days” as we experience them as artificial lighting tends to elevate cortisol. This can absolutely buggar sleep and crack open a host of problems with regards to body-fat levels, insulin resistance and systemic inflammation. This paper
is but one of many linking sleep debt and a disruption to the neuroregulation of appetite.
The take away from this is that our normally hard-wired sleep patterns require a significant amount of rack-time to properly experience. I don’t know ANYONE (myself included) that is going to bed when the sun goes down. So, we experience a compressed sleeping window and sleep that does not look like our ancestral/phenotypic norm. The BBC piece is a bit disturbing in that they essentially make the point that if you wake during the night it’s “nothing” to worry about because that’s a normal aspect of normal sleep. Do you see where this is flawed? We don’t GET normal sleep, so we need to do the best we can to get the sleep we need under the parameters of our lives. The researchers and journalists should be educating people to get more sleep, more dark time. They are not. They are taking cues from our normal sleep environment and overlaying this pattern on our current pattern which is a stress response to a compressed sleeping window and problems with photoperiod, both with regards to intensity of luminosity, and the problems associated with artificial lighting.