With summer's hot nights comes the seasonal battle for enough shut-eye. What can we do to beat the heat? Jamie Morton puts three questions to leading Massey University sleep researcher, Dr Karyn O'Keeffe.
Why does the heat beat sleep?
Here's an-all-too familiar scene.
It's 3am and the sheets you kicked off hours ago are a crumpled mess next to your bed.
You stumble bleary-eyed to the bedroom window - having decided you'll live with invading mosquitoes and the din of cicadas if that means getting any sleep before sunrise - and open it to find it's still hotter outside than in.
You shut the window again, head to the fridge for some chilled water, open your phone and see half the friends in your chat groups are online too.
Tomorrow is going to be tough, but at least you won't be alone.
What's to blame?
You might think it's the heat - especially when it's humid, too - but the real culprit is our circadian body clock.
This natural clock, found within the brain's hypothalamus, drives the 24-hour patterns of many biological processes, including sleep and wakefulness.
It also regulates something else that's key to healthy sleep - our core body temperature.
While this can vary from person to person - some of us do run hotter than others - the human body generally maintains its core temperature within a very narrow range of 36.5C to 38.5C.
Thanks to the circadian clock, that core body temperature sits around 1-2C higher in the late afternoon and early evening than during sleep, when it's at its lowest.
The average person's circadian cycle is characterised by a drop in core body temperature of about 0.2C to 0.5C around an hour before usual sleep time, falling to its lowest level between the middle and later span of night-time sleep.
It then begins to rise, acting as a kind of a biological alarm clock wake-up signal.
The temperature cycle leads the sleep cycle and is an essential part of getting to sleep quickly - and then snoozing well.
"There are universal things that we all share when it comes to good sleeping environments - we all like to sleep in the dark, in quiet, cool rooms," said Dr Karyn O'Keeffe, an associate director of Massey University's Sleep/Wake Research Centre.
"But, with what temperature is right, it can depend on the person. You might share a bed with someone who kicks off the blankets while you're trying to sleep with them on, or vice-versa.
"There appears to be quite a wide range of ambient temperatures that people prefer, from about 15C to 23C."
Getting to sleep - or just staying asleep - obviously proves tough when the ambient temperature soars above this range.
"We essentially need to shift our core temperature to our extremities to better fall sleep and stay asleep - so we need to cool our bodies by about 1-1.5C," she said.
"If we can't do that successfully, we toss and turn and lie there and feel frustrated."
Compounding the problem is the fact homes in the North Island generally have poor insulation, which makes them cold in winter, and too hot in summer.
"The other problem is that, over the summer break, if we're lucky enough to get one, we tend to throw our sleep patterns out the window," she said.
"We stay up later, or we sleep in - and then all of a sudden we have to go back to work or school again, and that leaves us feeling jet-lagged."
Of course, heat isn't the only thing that can stymie sleep.
"Stress is a considerable one - many peoples' financial situations have changed with the pandemic - and another is using bright, light-emitting devices in the evenings."
Originally, it was thought our circadian body clocks could only be adjusted by photoreceptors in the human retina that allow us to see, called rods and cones.
But now we understand that there is another part of the eye, called retinal ganglion cells, which include a subset that are directly photoreceptive themselves and are particularly sensitive to blue light.
Scrolling on a blue light smartphone screen can effectively reset our internal circadian body clock to a later time, postponing the release of sleep-promoting hormone melatonin, meaning we find it tougher to get out of bed when the sun is up.
"There's definitely a lot of evidence now to show that if you're using social media close to bedtime, that you'll have poor quality sleep."
That's why companies such as Apple have moved to "orange" screens at night, and many companies are now selling orange-enhanced night lights.
What happens when we can't sleep?
Ever wonder why we get constant mind-blanks the day after a bad sleep?
When we're sleep-deprived, our brains experience a build-up of neurochemicals that slow the firing rate, or activity, of neurons and hence promote sleepiness.
When this kicks in, we get periods of blankness, or "micro-sleeps", where we momentarily disengage from the world.
If we don't sleep enough, the neurochemicals that help us slumber simply keep working the following day, when we should be riding those that promote wakefulness.
Even without night-time heat, research tells us Kiwis aren't getting the shut-eye we need, anyhow.
One recent University of Auckland paper suggested that nearly 40 per cent of Kiwis are spending fewer than seven hours on the pillow each night – short of the optimal seven to nine hours.
With constant sleep deprivation and disruption come risks of high blood pressure - but even short-term sleep disruption brings a higher risk of weight gain.
Experimental studies have suggested this is because the hormones that regulate our appetite change, altering our levels of fullness and making us more likely to reach for those heavy, carb-loaded foods that pack on the kilograms.
"It can change our nervous system behaviour, which in turn can change the way we process glucose, or can change hormones that control what we eat," she said.
"Not only obesity, but also type-two diabetes, stroke and cardiovascular disease have been associated with poor sleep. It paints a pretty grim picture."
Sleep loss makes some jobs harder to do than others. People in creative jobs, for example, find themselves less productive, less able to problem-solve, more fixated on single ideas and struggling to think outside the box.
Others who work in potentially dangerous situations - a construction worker or a truck driver - need to know being tired could cause them to take more risks and make poorer decisions.
"Effectively, people aren't as clear-headed and they don't communicate as well," she said.
"It's well known that if people don't understand what others are saying to them, because that information's being processed differently, it can lead to accidents and injuries."
Sleep loss remains the most common cause of driver fatigue. In 2014, it accounted for more than 500 crashes on our roads - 31 fatal - and around $268m in related costs.
Road crash researchers point to people driving long distances without pulling over, and also to workers clocking up too much overtime before climbing into the driver's seat.
"Mood also seems to change quite a lot: we are more up and down, and we can't react appropriately to challenging situations," she said.
"Where we might usually be cool, calm and collected, that's hard when we're not getting enough good quality sleep."
We do have the option of topping up our sleep in the weekends, or when we can, just like paying off a credit card in overdraft.
Experts recommend catching up where possible, but it's most important to get enough sleep in every 24-hour period.
So what can we do?
If it's just that seasonal warmth that's stopping us slumbering, there are a few science-based hacks to try.
Trying to get to sleep two hours earlier than normal can be virtually impossible, as it goes against the drive of our circadian body clock, and only leaves us feeling more frustrated.
For those who don't have air conditioning, opening a window or turning on a fan are obvious options.
Another tip is taking a warm shower or bath at least an hour before bed.
That's because the process of heating up and then cooling down afterwards can mimic the drop in body temperature that we experience as we get sleepy in the evening and eventually drop off to sleep.
One US study suggested the optimal timing of bathing for cooling down our core body temperature to improve sleep quality was about 90 minutes before going to bed.
Warm baths and showers stimulate the body's thermoregulatory system, causing a big increase in the circulation of blood from the internal core of the body to our hands and feet - resulting in a shedding of body heat and a decline in body temperature.
Therefore, if baths are taken at the right biological time - one to two hours before bedtime - they'll aid the natural circadian process and increase our chances of not only falling asleep quickly, but also of getting better quality sleep.
"You don't want to get yourself too hot before bed, but a shower at moderate temperature can really start that cooling process," O'Keeffe said.
"There are some other tricks out there like putting your pyjamas in the freezer, or using chilled hot water bottles, but these aren't well studied."
And otherwise: stress less, keep healthy - and leave that smartphone on the night-stand.
Eight hacks to sleep better this summer
1. Freeze your sheets
Try sticking your bed sheets in the fridge or freezer for a few minutes before bed, placing them in a plastic bag first for temporarily cold bedding.
2. Sleep solo
Kick your partner out of the bed, cuddling increases body heat so sleeping alone will help prevent sticky, sweaty nights.
3. Cool showers
Take a cold shower just before bed, this will help bring your core body temperature down and you can hit the hay feeling clean and cool.
4. Close the curtains
Thirty per cent of unwanted heat comes from your windows, so closing them and shutting the curtains can lower the temperature.
5. Ice your fan
Place a bowl or pan of ice in front of a plug-in fan - the airflow generated by the fan will be even colder after it sweeps over the ice.
Don't forget to keep some water by your bed, dehydration makes it hard to regulate body temperature and sipping water throughout the night can help.
7. Use your heat pump
Heat pumps can also be reversed into air conditioners, these can be used to cool down the house very efficiently if the filters are clean.
8. Natural fibres
Ditch all your synthetic bedding like polyester for breathable bedding made from linen and cotton.