Debunking myths about daylight saving time: ten things everyone should know about the benefits of permanent standard time

State and federal governments are actively debating bills to replace clock change with either permanent daylight saving time (pDST) or permanent standard time (pST). In March 2022 the American Academy of Sleep Medicine updated its position on pST to clarify that pDST would be worse than continuing with seasonal DST. The medical community needs to understand the health and societal implications and get involved in advocacy to urge state and federal legislators to support pST. Here are 10 things everyone should know about DST.

BIANNUAL TIME CHANGE IS UNPOPULAR

Approximately 75% of Americans favor ending the twice-yearly switch to and from DST.¹ Adverse outcomes in the days and weeks after the “spring forward” change to DST include reduced sleep duration (15–30 minutes/day) and increases in heart attacks, strokes, atrial fibrillation, depression episodes, missed medical appointments, and motor vehicle crashes.^1,2 Students’ and athletes’ performance worsens and stock market variability increases. Minimal adverse effects are seen after the “fall back” change back to ST, which is more aligned with our circadian rhythm.

STANDARD TIME BEST ALIGNS WITH CIRCADIAN RHYTHMS

Although some may enjoy DST for the increased evening sunlight, later sunrises and sunsets are not aligned with our natural circadian rhythm. During ST, the sun is overhead closer to noon. Later clock times for sunrises and sunsets during DST delay circadian rhythms, which can make falling asleep more difficult, whereas wake-up time, as determined by social schedule, shifts an hour earlier compared to sun time.³ The negative effects of DST on sleep persist beyond the transitional days because circadian rhythms remain essentially aligned to solar time despite the new clock time. People may perceive these changes as problems with insomnia or sleep deprivation. These difficulties are actually due to circadian rhythm disturbances, by increasing social jet lag (difference in sleep times between free and work/school days) and shifting the midpoint of sleep later on free days.⁴

BEWARE OF CONCLUSIONS THAT SHORT-TERM EFFECTS MEAN pDST IS BETTER

Studies of clock change transitions are not designed to look at the year-round, long-term effects of pDST. Thus, they should not be used to support a position that pDST is better than seasonal clock change. For example, the assertions that pDST would be better for crime, mood, and activity levels (and thus health) are based on short-term transition studies.^5–7

Instead, studies that best inform the choice between pDST and pST provide information on long-term effects of temporal misalignment. These include (1) position within time zone studies, (2) natural experiments in locations that have changed time policies, and (3) data from populations with delayed circadian rhythms, including evening chronotype and social jet lag.

THE HEALTH EFFECTS OF DST PERSIST BEYOND THE CLOCK CHANGE

Later sunrises and sunsets, as seen with pDST, are associated with multiple long-term health outcomes including increases in stroke, heart attacks, diabetes, obesity, and cancer.^8–10

The increase in obesity and other cardiovascular health problems show that even if DST were to encourage exercise in some individuals it is not enough to outweigh the effects of sleep and circadian disruption on food choices, metabolism, hunger hormones, and glucose control and other cardiovascular risk factors.^11,12

pDST WOULD WORSEN SEASONAL AFFECTIVE DISORDER

In the winter, DST would delay sunrises to after 8 am for 2–4 months in the United States, with some locations only seeing sun after 9:30 am. Morning light is critical for resetting the body clock to the 24-hour day.

A Russian study evaluated a 7-year period comparing seasonal DST to pDST and pST.¹³ The study found the highest rates of seasonal depression in adolescents during pDST and lowest rates during pST. This aligns with known genetics of depression and effects of light therapy on mental health disorders that benefit from morning light and can worsen with evening light.¹⁴

THE ECONOMIC IMPACTS OF pDST ARE LIKELY WORSE THAN THOSE OF pST (AND EVEN WORSE THAN THE STATUS QUO)

Around the days of clock transition, DST has been found to boost retail spending because there is more evening light. A JPMorgan study found that DST was also associated with a relative increase in credit card spending, including higher utility and gasoline bills and health care spending.¹⁵ Although DST may save light energy it increases heating and cooling costs.^16–18

In terms of the overall workforce and productivity, the sleep and circadian rhythm disruption from pDST would negatively affect many outcomes related to performance and safety, including alertness, reaction time, procrastination, learning and memory, judgment, communication, creativity, and multitasking, and workplace injuries.^2,19 Circadian misalignment consistent with pDST is associated with losses in productivity, missed workdays, and lower average salaries.⁸

THE NEGATIVE IMPACTS OF CIRCADIAN MISALIGNMENT ON SAFETY AND CRIME RATES DURING pDST OUTWEIGH ANY POTENTIAL BENEFITS OF EVENING LIGHT EXPOSURE

One example of this misconception is the argument that pDST would be the safest option because evening light would reduce motor vehicle crashes. Some short-term data suggest that around clock change later sunsets decrease the number of evening commute-time motor vehicle crashes. A 2022 study, however, demonstrated people living in locations with at least 30-minute later sunrises and sunsets than at the time zone meridian have a 21.8% higher motor vehicle crash–related fatality rate.²⁰ This suggests that long-term effects on drowsy driving and other risky driving behaviors as well as the longer total commute time in the dark make pDST less favorable.

Data from studies comparing early and late school start time support how social policy changes that result in small improvements of sleep and circadian alignment can reduce crime and delinquency. Additionally, negative impacts from year-round circadian misalignment on factors such as sleep deprivation, judgment, and risk-taking behaviors may increase criminal behavior and substance abuse.²

pDST DOES NOT AFFECT THE POPULATION EQUALLY, LEADING TO STRUCTURAL DISPARITIES

Teenagers and young adults are particularly susceptible to delayed clock times and are most likely to experience sleep loss and social jet lag with DST.^4,8,13 Additionally, early work or school start times and/or chronic sleep loss result in greater risk of sleep disruption (Table 1)⁸ and are more likely in minorities and people with lower socioeconomic status.²¹

Table 1.

Factors affecting sleep loss with 1-hour-later sunrises and sunsets.

Social Factors	Average Daily Sleep Loss
Employed adults	19 minutes
Work starts before 7 am	36 minutes
Parents of children with school start time before 8 am	27 minutes
Work starts after 8:30 am without children	No loss

One example of these disparities was found in a 10-year study of standardized college entrance exam scores comparing students who resided in parts of Indiana on pST vs seasonal DST.²² Lower average test scores were found in the DST locations and lowest-income students were disproportionately affected (49 points lower in students from households with lowest income compared to 8 points lower in highest-income households).

pDST HAS BEEN TRIED BEFORE AND QUICKLY BECOMES UNPOPULAR

The United States tried pDST twice before, most recently in 1974, which ended early because it was unpopular. The observation of increased deaths of school children at dark morning bus stops is cited as a reason for ending the practice. Similar unpopularity occurred when Russia adopted pDST from 2011 to 2014, eventually reverting to pST. Notably, 60% of countries follow pST (Figure 1).

Figure 1. Only 40% of countries use seasonal daylight saving time (in blue).

ADVOCACY IS NEEDED NOW

The Sunshine Protection Act, which proposes federal pDST, passed the US Senate in 2022 but failed to pass the House. It has subsequently been refiled in the Senate in 2023. pST is already federally approved, but we need a federal bill to ensure public health and safety by applying pST as a standard across the United States. Although a federal bill would be best to coordinate times nationally, state bills, even if dependent on passage of bills in neighboring states, are needed to show that people want pST, given that 19 states have already passed bills that would allow for pDST.

The Society for Research in Biological Rhythms,⁴ American Academy of Sleep Medicine,^23,24 Sleep Research Society,¹ American Medical Association,²⁵ and American Academy of Neurology are some of the many medical and research societies that have endorsed pST.

We urge you to take an active role in educating the public and advocating by contacting your national and local congresspeople to block pDST bills and file pST bills instead. You can contact your representatives using https://aasm.org/advocacy/take-action/; for more information see https://www.savestandardtime.com/ and the educational video series “The Science of Clock Change” at https://www.youtube.com/@SaveStandard.

DISCLOSURE STATEMENT

All authors have seen and approved the manuscript. Dr. Johnson is vice president of Save Standard Time, a volunteer-run nonprofit supporting permanent standard time. The authors report no conflicts of interest.