Deep dive: Daylight savings time
Deep dive: Daylight savings time
How DST affects our health — and ways to compensate
I write this from India, where we’re visiting family and attending a wedding — a country where the clocks stay put year-round. But I know that daylight savings time (DST) was just observed in the US this past weekend — and many of you are feeling its effects. From here, all DST means is a slightly more favorable time difference overlap for long-distance family calls between the US and India.
It turns out that three-quarters of the world’s population does not observe this biannual switch between DST and standard time — including India and the Philippines, two countries I spent time in as a child, along with much of the rest of Asia, Africa, Latin America, and the Caribbean.
Within the US, Hawaii, parts of Arizona, American Samoa, Puerto Rico, Guam, the Northern Mariana Islands, and the US Virgin Islands don’t observe this custom, either. The rest of the US does.
Benjamin Franklin wrote a satirical letter in 1784 proclaiming that waking people up earlier in the summer — creating more sunlit evening time — would be a great way to save on candle use.
But DST was first introduced in the US by President Wilson in 1918 on the tail end of World War I as a way to extend daylight and conserve fuel, modeled after Germany and other European powers. But it proved unpopular and was abolished federally by Congress, though it remained a local option. It was next reinstated as a year-round policy (called “war time”) by President Wilson during World War II, from 1942 to 1945. The twice-yearly clock change, with DST only during the summer, started in the following year, with local variations in timing until the Uniform Time Act standardized DST across the country in 1966.
The commonly held explanation that DST favors farming needs turns out to be incorrect — farmers have lobbied against DST since its inception, as their immediate tasks are often dictated by the sun and therefore adversely impacted by DST.
From a 1942 TIME article on DST:
As in 1917, when the U.S. first adopted daylight time as a war measure, farmers were the loudest objectors. The cow, they cried, is a delicately balanced creature, yields less milk for defense when her hours are disturbed. The dew, they insisted, stays on the grass until 9 a.m. (10 a.m. daylight saving time), and farmers cannot work their fields until the dew dries. Rising before dawn, they declared, they would be dog-tired long before day's end.
All that said, when I arrived in the US, as someone unused to it growing up, the switch was jarring every March and every November.
During medical training, ‘falling back’ often meant working an extra hour during an already stressful night or 28-hour shift. And ‘springing forward’ to DST? A lost hour of much needed sleep. But the impacts actually go much further — aside from health consequences (see below), both switches incur disorientation and impaired executive functioning — impacting patient safety and outcomes. The week following either time switch carries an increased rate of human errors in medicine, by about 19% for DST and about 5% for the standard time switch.
Understandably, there was some public joy at the idea of fixing our clocks in place when last year, the Senate unanimously approved the Sunshine Protection Act, to switch permanently to DST — and do away with this twice yearly switch, rife with inconvenience and goof-ups, like missed appointments and household clocks that stay stuck an hour off the new time for a while thereafter.
The US actually did try a switch to permanent DST in 1974 as an energy-saving mechanism in the wake of the energy crisis of the 1970s. It was unpopular, especially after several publicized early morning car accidents killed children. And the Department of Transportation reported minimal benefits from the standpoint of energy conservation. (A recent analysis found savings on electricity of about 0.3% from DST.) The change was repealed just over a mere 10 months later.
The science says: Staying permanently in DST — rather than standard time — would desynchronize our natural or internal circadian rhythms from those of the sun, because we’d lose morning sunshine hours (we’d wake up to darker mornings) and gain exposure to brighter evenings (the sun would set ‘later’ in the day).
This is the opposite of what our brains and bodies need. We do best with early morning sunlight and darker evenings — and DST inhibits this.
Our internal clocks are set by well-timed exposure to sunlight and darkness, acting in part through a hormone called melatonin (suppressed by light and active in the dark, triggering us to sleep) — in addition to other cues like food, physical activity, and social inputs.
Exposure to early morning light coinciding with our awakening (ie: when sunrise is closer to our wakeup times) and to darker evenings (when sunset is a few hours prior to bedtime) is optimal.
This circadian system is intrinsically tied to a number of essential functions, not just our sleep, but also our metabolic processes, our hormones, our immune systems, and our moods and cognition.
So how exactly does DST disrupt these systems?
DST makes it so we take longer after rising to achieve optimal alertness (and associated functions) and on the other end, are unable to go to sleep ‘on time,’ given the extra evening light exposure, which suppresses melatonin and other sleep-regulating systems — but still have to wake up early (earlier than ideal for our internal clocks). In technical terms, DST induces “phase delay,” so our internal circadian clock, set by the sun, is off-cycle — later — relative to the time by the clock. These effects are particularly felt in Western-most areas of any time zone, because the sun already rises and sets at a later time by the clocks there.
Here’s how DST affects our health:
The DST switch results in brain and bodily changes on the level of altered molecular signaling and gene expression, including in the genes governing our internal clocks and sleep, stress responses, functioning of the heart, immune and inflammatory systems, and several hormonal cascades. Chronic circadian misalignment increases inflammation and stress reactivity, while decreasing parasympathetic nervous system tone (which helps us rest and digest), with increased heart rate and lower heart rate variability — all setups for acute cardiometabolic events and potentially longer-term health risks.
Every year, in the days immediately after DST goes into effect, we see an increase in acute events like heart attacks, cardiac arrests, strokes (particularly for women, those over 65 years, and those with cancer), and the heart arrhythmia atrial fibrillation (particularly for women). Heart attacks have been observed to fall during the transition back to standard time.
In the week after the shift to DST, we see an increase in the rate of fatal car accidents by 6%, particularly in the morning and in westward locations (which have greater circadian misalignment) — but after a few weeks, total car crashes actually decrease, presumably because of brighter evening driving conditions, by one estimate by 18% over the eight weeks following the DST change.
In a period of 10 weeks following DST in Denmark, depression increased by 11%. DST also works against the internal mechanisms that help us adjust to the seasonal changes in sunlight exposure and its effects can persist for months. The change to DST has also been associated with missed outpatient medical appointments (not observed with the switch back to standard time) and an increase in acute or emergency visits, as well as return visits (within 4 days of the first visit).
What’s the economic impact?
Aside from the health consequences above, DST acutely decreases worker productivity and increases workplace injury — by one estimate, DST actually costs the US $434 million every year. (But that extra hour of evening light is profitable for certain key lobbies, including leisure goods and services, sporting goods and events, gas stations and convenience stores — central to why it persists.)
Here’s the bottom line:
“There is evidence that the body clock does not adjust to DST even after several months. Permanent DST could therefore result in permanent phase delay, a condition that can also lead to a perpetual discrepancy between the innate biological clock and the extrinsic environmental clock, as well as chronic sleep loss.”
—American Academy of Sleep Medicine’s position statement on DST
Many public health groups, including the American Academy of Sleep Medicine, have issued statements disavowing a move to permanent DST on the basis of its health impacts — and instead support instating permanent standard time.
Mexico actually did choose standard time as their permanent option just last year (2022) — so stay tuned for studies on medium and longer-term impacts from this policy change.
Luckily, in the US, permanent DST didn’t gain favor in the House last year. The Department of Transportation is due to issue an updated report studying potential impacts by December 2023 — so we might expect Congress to reconsider this question in 2024 or thereafter. When that happens, urge your Congresspeople to consider standard time instead.
What can we do about it?
So what does all this mean for us now, in the days to come? How can we best counteract the effects of the DST switch on our health? (This is a light version of what I do when I cross 10-12 time zones from India or the Philippines to the US and back.)
1. Honor your internal clock. If your schedule allows, fall asleep and stay asleep at the times most comfortable for your body (synced to the rhythm of the sun) — and try to keep this similar across most days. Expose yourself to morning sunlight, preferably outdoors. If you wake up or start work or school before sunrise, use a dawn simulator or bright light therapy. At night, dim the lights and try to stay off screens for at least a couple of hours before bed. Consider using blue-light blockers to minimize the brain-awakening effects of evening light, including outside.
2. Emphasize anti-inflammatory nutrition. Knowing that these weeks may induce extra inflammation as your body settles into DST, take extra care to choose foods that help decrease inflammation. Reach for whole grains over white or processed items; plenty of fruits, vegetables (frozen or fresh!), nuts, cheese, and lean proteins; and water over soda, juice, or alcohol.
3. Physical activity. As school and work routines permit, consider getting in a walk or workout at the beginning of your day most days of the week — ideally outside so you give your body multiple ‘wake up’ cues at once. This will help with regulating stress, metabolic processes, energy, sleep, cognition, and mood.
4. Connection. Connection is a hugely helpful antidote to systemic dysregulation — make the extra effort to stay connected to those who are important to you, either in person or remotely. During DST, there’s a tendency with later sunsets to also elongate our social engagements and push back our bedtimes — but getting those crucial hours of sleep every night is extremely stabilizing.
5. Be in the moment. Practices that ground us and help us be in the present moment are also key for reducing the impacts of stress, inflammation, and metabolic dysregulation. That might take the form of laughter, yoga or meditation, art, music, a spiritual practice, or gratitude journaling — or something else. Find whatever works best for you.
Be gentle with yourselves and each other as we all adjust to this paradigm over the next few weeks. Physiologically, the DST switch can indeed be hard — so keep that in mind if you feel more reactive or stressed, and give yourself grace and time to adjust. We’ll get through this.
We’d also love to hear from you all in the comments — what’s worked for you in riding out DST?
Wishing you light,
Dr Devika Bhushan
Mark your calendars!
Our first ever ‘community corner’ chat will take place on March 31, at 11:15 to 12 noon PT and 2:15 to 3 pm ET.
Open to paid subscribers, this chat will be a chance for us to dive deeper into key themes and questions raised by recent newsletters, get to know our community, and benefit from each others’ lived experiences and wisdom.
A note on the next edition:
The next edition of Ask Dr Devika B will mark World Bipolar Day on Thursday, March 30. So it’ll land in your inbox that Thursday rather than the Tuesday of that week.