Goodnight, Sleep Tight: Slumber in the Big City

On a pleasant autumn evening, I found myself amidst the large, ancient boulders of Kolar. Red gneissic rocks stood tall like windowless skyscrapers, with shrubs and the occasional tree growing at their base. A short drive away from Bangalore city, the hillocks of Kolar stood in stark contrast to the metro, seldom intruded upon by humans and far removed from the bright lights of the city. I held up my lux meter to check the light intensity. Zero lux, the device blinked back. Holding one below a streetlight could record a blinding 70 lux. And this lack of disturbance in the landscape from lights meant one could study how animals slept in natural conditions. Turning our head torches on, my research team scanned the ground, shrubs, and rocks, looking for animals who had turned in for the night, in particular the South Indian rock agama (Psammophilus dorsalis).  

A good night’s sleep is sought after across the animal kingdom. In the wild, a safe place to sleep that keeps you comfortable from the elements and away from disturbance is all the more important. When the night is crawling with predators, where one chooses to sleep can be the difference between life and death — a place unprotected from rains could cause hypothermia, and poor sleep caused by disturbances (like bright lights) can lead to a drop in reaction time against threats. Those who cease to function or function with a steady dose of grumpiness after a night of poor sleep will readily testify to the importance of sleep for our survival. Humans, knowingly or not, choose to sleep in places that fulfil these basic requirements of safety and comfort — a bed raised from the ground, behind securely locked doors, in regulated temperatures, in darkness. Animals can be as particular in choosing their sleep sites. And animals that have managed to survive in cities provide us with an opportunity to understand how sleep changes under urban conditions of bright lights and high decibel levels.  

The South Indian rock agama (Psammophilus dorsalis) rules over the rocky terrain of the Indian peninsula. Its bright red, orange, and black colours stunning against the brown landscape. This diurnal lizard has also managed to survive in the metro of Bangalore and is commonly seen in gardens and unbuilt parcels of land. A body of scientific research has uncovered several reasons why the lizard might be particularly resilient to the challenges thrown by city life, whereas other species are not so successful. The tenacious city populations of the species have learnt to fine-tune their escape strategy (when and where to run), display appropriate colours, and be less aggressive. This understanding, however, is incomplete without knowing how their lives change when the sun sets, and the city lights rise.

These lizards are particularly susceptible to light pollution as they possess a light-sensitive pineal eye (third eye). Remember the annoyance of having a bright light turned on when we are asleep? Having a pineal eye makes it much worse. Do agamas from cities, faced with altered habitats, bright lights, and a suite of novel threats (such as feral cats), then sleep differently from their rural counterparts? Knowing how sleep sites differ between natural and disturbed places allows us to better understand how animals can change their sleep behaviour according to the environment they find themselves in. And eventually, knowing their sleep ecology may shed light on why some species fare better in cities and similarly challenging places than others. 

In our quest, we roamed the brightly lit by-lanes of Bangalore at night, searching for snoozing lizards. While our head torches had illuminated the boulders and ledges of Kolar and Avathi, here, they shone on concrete walls and plots overgrown with shrubs. While the conundrum was to climb precarious rocks in natural sites, here we hesitated to jump into empty plots littered with trash. Sampling next to people’s homes also meant getting hollered at by startled residents spooked at lights scanning their property’s perimeter. But once we communicated our seemingly outlandish plans to them, we were met with curiosity and sometimes concern for our well-being while trampling through the overgrowth. Once we spotted a sleeping lizard, we proceeded to collect a suite of measurements of the lizard and the sleep site. For each sleep site, we characterised the structure (substrate type) and light conditions (covered or not, light intensity). Three hundred and two lizards later, a clear picture emerged on sleep site choices of rock agamas in rural vs urban areas. 

In rural sites, agamas slept on rocks, clinging vertically to the coarse surface. In the city, rocks were replaced by unplastered concrete walls. Such a seemingly uncomfortable sleep posture likely grants the lizard protection against predators, who cannot match their climbing skills. We found that city agamas were nine times more likely than their rural counterparts to sleep in covered sites, sheltered against light, ensuring dark spots to shut eyelids under. A covered site would typically be a rock (or concrete wall) covered by an adjoining shrub and on occasion a tarpaulin sheet or other artificial material. Such a drastic shift (to seek darkness) in sleep behaviour in the face of an altered world is not only a testament to the remarkable behavioural flexibility of these agamas, but also to the intrinsic motivation in life forms to ensure they sleep unperturbed. Humans change their immediate environment to best suit their requirements, which in turn may change how animals use the same places. Streetlights increase the areas of safe access humans enjoy, but these can also render the same places inaccessible to others. On the other hand, letting the perimeter walls of houses remain unplastered allow lizards to find rocky sleep sites of their choice.

Apart from rock agamas, several other animals try to find ideal sleeping spots in a city. A clump of grass may be ideal for the common garden lizard (Calotes versicolor) to sleep peacefully, whereas Asian vine snakes (Ahaeutlla prasina) may need leafy young plants. While some species need vegetation overhanging waterbodies, some need to shelter under rocks that are not dislodged. For humans, poor sleep quality is no less than a local epidemic in cities, owing in no small part to the bright lights (particularly high-intensity white light) we surround ourselves with, that alter our levels of sleep-promoting hormones. While the rock agamas of Bangalore can avoid these lights to a certain extent, their sleep quality and those of other animals may be poor in cities. Understanding when, where, how, and how much animals sleep in cities, may hold lessons for our sleep health. When a good night’s sleep beckons me, I often wonder if life around me is also safely tucked in for the night.    

Photo source (rock agama), Photo source (rock agama on concrete)