Lara Weed

Time Policy Map — Lara Weed

How is your state and county circadian health impacted by Standard Time, Daylight Savings, and Biannual Switching time policies in the US?

You can find out with our (now mobile-friendly) Time Policy Map!
www.laraweed.com/time-policy-...

Switching from the United States’ current policy to Standard Time reduces overall obesity prevalence more than switching to Daylight Saving Time.

Switching the clocks twice a year hurts Americans' health. A modeling study of county-level solar light patterns, time policy, health data, and circadian models suggests a permanent shift to Standard Time would reduce obesity and strokes. In PNAS: www.pnas.org/doi/10.1073/...

Hate Daylight Saving Time? Our Body Clock Might Prefer Permanent Standard Time A new study suggests that permanent standard time may reduce obesity and stroke

A new study suggests that permanent standard time may reduce obesity and stroke

Hate Daylight Saving Time? Our Body Clock Might Prefer Permanent Standard Time A new study suggests that permanent standard time may reduce obesity and stroke

Glad to provide commentary about the new PNAS modeling article by @laraweed.bsky.social about how permanent standard time vs. permanent daylight saving time vs. biannual switching impacts population level health by county. @savestandard.bsky.social

www.scientificamerican.com/article/nixi...

Work from our lab on the circadian health impacts of time policy!

Circadian-informed modeling predicts regional variation in obesity and stroke outcomes under different permanent US time policies | PNAS Seasonal changes in time policy, such as switching between Standard Time (SDT) and Daylight Saving Time (DST), have been adopted by many countries,...

A simple way to help reduce obesity and stroke?
Stop the shifts in our clocks twice a year
www.pnas.org/doi/abs/10.1...

The solstice does not always align with the earliest sunrise and latest sunset of the year. This year in Palo Alto, earliest sunrise was on June 13th and the latest sunsets will be on June 26th & 27th, but the solstice is on June 20th.

Around the solstice, the sun’s position at solar noon barely changes day to day. Early observers noticed this and aptly named it solstice, from the sun (𝘴𝘰𝘭) standing still (𝘴𝘪𝘴𝘵𝘦𝘳𝘦).

It’s solstice week.

The longest stretch of daylight of the year happens in the Northern Hemisphere this Friday.

At 7:42 PM PT, the sun reaches its maximum northern declination, marking the summer solstice.

Sleep and circadian rhythms are connected, but not the same. While sleep timing and architecture can be influenced by circadian, homeostatic pressure is the main lever. Recognizing the difference helps explain jet lag, shift work, and why good sleep isn’t just about duration.

Sleep and circadian rhythms respond to different inputs. Sleep pressure builds with time awake and can be relieved by sleep or naps. Circadian rhythms respond to light. Optimizing means using both inputs to align your physiology.

Exercise increases the rate of sleep pressure build up. This is part of why you may want to go to bed earlier after a hard training day.

A long nap can make it harder to fall asleep at night. Naps reduce sleep pressure, which can delay sleep onset, even if your circadian rhythm is promoting it. Sleep regulation is a complex interaction between circadian, sleep pressure, and prior sleep.

Many sleep studies don’t fully separate circadian timing from sleep pressure. Without careful control, it’s hard to tell which system is driving the effect. That’s why protocols like constant routines and forced desynchrony matter so much.

One of the strange things about studying sleep: there’s no such thing as a “sleeping cell.” Sleep is an emergent physiological and behavioral state. Reductionist tools still matter, but they don’t capture the phenomenon on their own. That’s what makes it so hard and so interesting to study.

The two-process model of sleep regulation is like a thermostat: sleep pressure builds like rising heat, and sleep cools it down. But the “on/off” threshold isn’t fixed. Instead, it’s set by your circadian clock. Timing determines when sleep becomes theoretically possible.

You can get ~7 hours of sleep and still have circadian misalignment. You can also be well-aligned and still be sleep-deprived. These are connected systems, but not interchangeable.

Sleep and circadian rhythms are often conflated on social media but they are in fact distinct. Sleep is considered a physiological and behavioral state, whereas the circadian rhythm is the body’s internal clock that keeps time, even in the absence of sleep.

Aligning to the outside world is important, but full circadian health takes more than light. It means timing your meals, movement, and routines to support internal synchrony. Your body runs best when its clocks work together to harness predictive physiology.

Circadian misalignment comes in two forms:

External - when your internal clock is out of sync with local time

Internal - when your body’s clocks are out of sync with each other

Both matter. And both can affect how you feel, sleep, and function.

You can experience chronic internal misalignment when your lifestyle repeatedly sends conflicting time cues to different parts of the body. Shift work, frequent time zone travel, and irregular eating or sleeping patterns can all lead to persistent internal desynchrony.

Social jet lag is when your sleep schedule shifts between workdays and weekends, like flying across time zones without leaving home. This common misalignment has been linked to metabolic issues, mood disturbances, and poor sleep quality, even in otherwise healthy people.

To reduce internal misalignment, align your cues. Eat at consistent times, avoid late-night meals, time exercise during the day, and anchor your routine to your light-dark cycle. Internal clocks sync best when signals from the environment and the brain agree.

Which realigns faster after travel: internal alignment or external alignment of the circadian system?

External. The central clock can shift ~1 timezone/day with light. Internal clocks (e.g., gut, liver) adjust more slowly, through central clock and cues (e.g. food, activity, and temperature).

Peripheral clocks, like those in your gut, liver, and muscles, take cues from the brain’s central clock, but also respond directly to other signals like food, exercise, and temperature. They can run on predictive physiology, or just react to the world in real time.

A lot of people focus on light to support their circadian rhythms (external alignment), and that’s a great start. But full-body circadian health also depends on internal alignment. Your gut, liver, and muscles need timed cues, too.

Your circadian rhythm doesn’t shift with your flight. It adjusts slowly (~1 timezone per day), leading to jet lag until your body resynchronizes with local time. Time your light and sleep to realign faster.

Jet lag creates two kinds of misalignment: external (your body is out of sync with local time) and internal (your organ systems are out of sync with each other). Both affect how you feel, sleep, and perform.

You can correct external misalignment by adjusting your light diet by timing light exposure to shift your internal clock toward local time. But just aligning to the environment doesn’t guarantee your body’s internal systems are in sync with each other.

External circadian misalignment happens when your environment shifts faster than your internal clock. It’s caused by things like jet lag, shift work, late-night light exposure, or social schedules that don’t match your biology.