Most animals sleep but scientists still lack a complete understanding of why, the biological factors that regulate sleep cycles and how the behavior evolved.

• A new study in Nature on tiny zebrafish finds how humans sleep today may have first evolved in vertebrates more than 430 million years ago.

Why it matters: Scientists hope a better understanding of how zebrafish sleep, down to the cellular level, could unlock avenues for new treatments of sleep disorders that affect 50-70 million Americans but are often undiagnosed. It could also improve their understanding of sleep's role in memory and our physical health.

What they did: Zebrafish lack a neocortex — the part of the mammalian brain involved in higher order functions such as sleep and the target of sleep studies — so the scientists from Stanford University and institutions in Japan and France needed to find a workaround.

• They developed an imaging platform that allowed them to view proxies for brain and muscle activity via fluorescence, as well as heart rate and eye movement.
• They used two-week-old zebrafish since they are transparent, and therefore ideal for fluorescent imaging techniques.
• They also performed other experiments to determine the effects of particular compounds on zebrafish sleep and wake cycles.

What they found: Ependymal cells — present in humans and zebrafish, and known to play a role in the brain and spinal cord — are among the first to be activated as the fish fall asleep.

• The researchers also found human hypnotics — compounds found in sleeping pills and anesthetics — can also induce sleep phases in the fish that are analogous to humans.
• And similar to what is seen in humans, chemicals released by neurons in the zebrafish brain regulate the creature's muscles and sleep.

What's new: Scientists had observed invertebrates (octopi, insects) and vertebrates (fishes, amphibians, birds, mammals) sleeping but the physiological signatures of sleep, like rapid eye movements, had only been observed in mammals, birds and reptiles, study co-author Philippe Mourrain of Stanford University tells Axios.

What they're saying: “You can’t just say sleep is sleep,” Jerry Siegel, a sleep scientist at UCLA who was not involved in the study, told National Geographic. He cautioned that connections between sleep in young zebrafish and sleep in humans are less straightforward than the study suggests. For example, he said, many mammals lack REM sleep completely, and some mammals sleep 20 hours a day whereas others just need 6 to 8 hours or less.

The study's lead author, Louis C. Leung of Stanford, tells Axios:

"I encourage everyone to cherish what has taken 100's of millions of years to create and hope there is soon a change in the public narrative regarding the importance of sleep — we should be proud not embarrassed to get enough sleep."

What's next: Developing animal models that examine sleep functions at the cellular level, like the one in this study, could lead to treatments for sleep disturbances that are linked to psychiatric disorders as well as Parkinson's and Alzheimer's.

Alaska's recent heat wave has been grabbing headlines, and rightly so: The state has been hotter than parts of the contiguous U.S. during June and July.

• Anchorage hit 90°F for the first time ever, and the heat plus summer thunderstorms have triggered massive wildfires across the state.
• An area greater than Rhode Island has burned in just the past 11 days.

But this isn't the whole story.

The big picture: The ongoing heat wave is part of a longer-term accelerated warming trend that is altering life for Alaskans, many of whom live in rural communities dependent on ice and snow cover for transportation and hunting.

By the numbers: The most recent heat has been staggering, but so too is the long-term picture. Alaska has now had its:

• Warmest 12-month period on record, (July 2018 through June 2019) since 1925.
• Warmest 24-month period (July 2017 to June 2019)
• Warmest 48-month period (July 2015 to June 2019)
• Warmest 64-month period (July 2015 to June 2019)

What they're saying: The duration of the heat, plus the shockingly rapid and early disappearance of sea ice from the Bering and Chukchi Seas is most notable, Rick Thoman, a climate expert at the University of Alaska, tells Axios.

Meteorologist Brian Brettschneider said the record heat would most likely not have happened without the overall, climate change-driven warming of the state.

"Weather happens on top of climate. An airmass similar to what we just experienced has occurred several times in the past. What's different now is that the very warm airmass was placed over an environment that has already warmed by several degrees," he told Axios.

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One of the most unusual cloud types Earth's atmosphere produces — noctilucent or "night shining" clouds — can be seen best at high latitudes in the summer. So far in the summer of 2019, such clouds have been showing up with unusual frequency and geographic coverage.

• Noctilucent clouds appear soon after sundown, glowing blue and white so high in the sky that they're lit by sunlight after the sun goes below the horizon.
• They can remain for hours after the sun sets, depending on the location.
• The clouds occur high in the atmosphere, about 50 to 52 miles up in a layer known as the mesosphere.
• For perspective, they are closer to the height of space tourism than they are to typical jet aircraft cruising altitudes.

How it works: These clouds are thought to form in the summer when Earth's lower atmosphere warms, but the upper atmosphere cools. This encourages ice crystals to collect on aerosols and meteor dust near the edge of space.

• Ripples in the atmosphere known as gravity waves are also thought to play a role in generating these clouds.

What's new: Scientists are studying noctilucent clouds to try to understand why they seem to be occurring with greater frequency, and showing up farther south than before.

• The composite image was made using data from NASA's Aeronomy of Ice in the Mesosphere (AIM) spacecraft, an instrument that measures the amount of light reflected back into space by the night-shining clouds.
• The researchers found the clouds are forming further and further south, and becoming more common — possibly due to increases in water vapor in the upper atmosphere related to climate change.
• Yes, but: Periods of low solar activity — and we are in one — can also spur more noctilucent cloud activity, since there's less UV radiation to break up water molecules.