Exposure to natural light is a critical ingredient for a good night’s sleep – but most people don’t understand that we need BOTH bright light during the day, AND biological ‘darkness’ to sleep well at night.  Blue light and sleep are fundamentally tied together – our bodies depend upon a bright blue-rich light signal during the day and the absence of  blue-rich light at night to regulate our sleep cycle.  Without enough light during the day, we don’t sleep well — with too much light at night we don’t sleep well.   Increasing our exposure to natural light during the day (get outside!), and reducing our use of artificial light at night are two of the simplest things we can do to improve our sleep.

Sleep is a complex process governed by many factors, but our daily cycles of awake/alert and recovery/rest depend upon well-defined daily cycles of light and dark.   Exposure to bright natural light during the day stimulates our energy, mood, metabolism and day-time hormones (like cortisol) while suppressing melatonin.   As many of us know intuitively, we sleep better after spending time outdoors.  Studies show clearly that without sufficient bright light during the day (which drives day-time melatonin suppression), we don’t sleep well at night.  One of the simplest things we can do to sleep better is to get outside for at least 20-30 minutes in the morning.  Darkness at night is critical for relaxation, sleep and our melatonin cycle (night-time recovery hormone).   Studies show that exposure to artificial light at night (fluorescents, LEDs, tablet and cellphone screens etc) inhibits sleep by delaying sleep onset, suppressing melatonin and stimulating our mind, body and metabolism.  Physicians recommend minimizing screen time and exposure to bright lights for about 2 hours before bed. Furthermore our chronotype (the timing of our sleep cycle) is strongly linked to exposure to light.  Studies have shown that camping for a week brings both early birds and night owls back to a naturally synchronized cycle.  Robust daily circadian rhythms are a cornerstone of healthy sleep and require both bright light during the day and natural darkness at night = light and sleep go hand and hand!

 

Bright light at night suppresses the production of Melatonin (our night-time recovery hormone).  Figure from JCM, for more context see this Tuck article on light and sleep.

Our circadian rhythms our synchronized by a third photo receptor in our eye (ipRGCs) that is sensitive to the intensity of blue light in our surroundings – for more on the underlying mechanisms which tie blue light and sleep together check out our Science page.   In simple terms, the presence of bright blue-rich light signals ‘awake and alert’ while the absence of blue-rich light signals ‘recover and rest’.   The timing of light exposure is also important, we are most sensitive to the presence/absence of light in the morning and in the evening.  Extra bright light before sunrise shifts our body clock forward (we wake-up earlier), while extra bright light after sunset shifts our body clock back (we go to sleep later).  Studies have established thresholds for biologically active bright light (>10,000 lux) during the day and have shown that as little as 10 lux is sufficient to disturb sleep at night.  Fortunately we do not have to forgo all light at night, the research suggests that we just have to be smart about what type of light we use after dark.  The warm light of a candle, fire, or a dim incandescent contains very little blue light and therefore does not stimulate the blue-sensitive ipRGCs.  From an evolutionary perspective this is not a surprise as artificial light has only existed for ~100 years, and most plants and animals depend upon the daily cycle of bright light and darkness to regulate their bio-rhythms.  For optimum sleep our bodies crave a return to natural lighting conditions, with exposure to at least 30 minutes of bright light (>10,000 lux) during the day, and only warm/soft light or darkness at night.