Body clocks all at sea

Listening out for the many rhythms of life

The thought of being at sea under a starry sky, charting a way through inky black waters with only the waves for company, may be a dream for some, but for those who work far from shore, the reality is often far from romantic. For the sea never stops. Forever in motion, it is in constant need of close monitoring, lest even one of the great many risks go unattended on a dark, churning night.

As land-dwelling creatures who evolved under the ever reliable 24-hour solar cycle, occupational demands at sea — round the clock and unrelenting — are a direct challenge to our finely tuned circadian rhythms. And once body and mind are out of synch, and tiredness sets in, the dangers of work on the waves only heighten. The recent incident whereby a cargo ship ran aground on Norway’s west coast, making a surprise entrance in a fjord-side front garden, is an unsettling reminder of the kind of trouble that fatigue on the job can cause.

Besides the biologically discombobulating shift patterns, life at sea presents a host of other sleep-inhibiting stressors: noise, vibrations and not least motion. Exposure to the sways and surges of a vessel for a prolonged period can apparently bring on Sopite Syndrome, from Latin sopire ‘lull to sleep’. Though this sounds pleasant, it is anything but — a dangerous if still poorly understood state of withdrawal, characterised by drowsiness, apathy and reduced alertness.

Sailors are not the only ones to face such challenges. Sleep disturbances and upturned biorhythms are also reported in the offshore energy sector, where workers live on perpetually producing oil platforms, typically for two weeks at a time, offset by four weeks’ downtime onshore. A study of 103 North Sea oil and gas workers found around a quarter suffering from Shift Work Disorder, a severe sleep disorder brought on by shift patterns out of synch with an individual’s usual sleep routine, while a vast majority of participants experienced some level of offshore sleep trouble.¹ Workers on the latest frontier of North Sea energy production — offshore wind farms — report similar problems.²

A North Sea oil platform at night (Olivier Dugornay, IFREMER, Pôle Images, Centre Bretagne - ZI de la Pointe du Diable - CS 10070 - 29280 Plouzané, France: CC-BY-4.00)

It strikes me as one of life’s ironic injustices that working lives at sea — an environment in such close proximity to nature — should be so at odds with the ingredients necessary to maintain our naturally evolved biorhythms. Of course, for those creatures who live their lives beneath the sea’s surface, things are rather different.

Waves and tides have their own periodicity and, just as we terrestrials are finely attuned to the solar cycle and its cues of light and dark, so too marine species are delicately adjusted to the natural cycles ticking over underwater. After all, it was in marine environments that life itself likely originated, so there’s been plenty of time for sea creatures to become entrained — the formal term for when a living organism synchs its inner clock with environmental time cues. (As a keen German-speaker, I am overjoyed to find that these external rhythmic cues are known in the academic literature as Zeitgebers, literally ‘time-givers’ — the beauty of Germanic compounding shines through as ever.)

The tidal cycle is one feature of this temporally complex marine realm. We are all familiar with the periodic rise and fall of the sea under the pull of the Sun and the Moon — at 12.4-hour intervals, twice per lunar day. What is less well known is that these tidal intervals serve as a biological metronome for many of our much-loved marine life forms, and some of our tastiest too. Patterns of cell renewal in mussels from the Bay of Biscay, for example, have been observed to increase during low tide and decrease during high tide — a circatidal rather than circadian biorhythm.³ Mussels from the tamer waters of the Mediterranean, meanwhile, in the obvious absence of any strong tides, take their metronomic cue from daylight.⁴

Mussels (Gil Ndjouwou on Unsplash)

But just like humans, marine creatures can also find their internal clock thrown out of synch. Artificial lighting on the sea at night, as well as noise pollution from underwater drilling, are among the key culprits here.

And so, as offshore activity in areas like the North Sea continues amidst a rapid energy transition, understanding the bioclocks of both humans and marine species in these dynamic environments seems more important than ever — another one of science’s little championed but highly relevant research areas, with much still to discover, even if the geophysical cycles and evolved biorhythms are themselves many millennia old.

1. Waage, S., Moen, B. E., Pallesen, S., Eriksen, H. R., Ursin, H., Akerstedt, T. & Bjorvatn, B. 2009. Shift work disorder among oil rig workers in the North Sea. Sleep 32(4), 558–565.

2. Velasco Garrido M., Mette J., Mache S., Harth, V. & Preisser, A. M. 2018. Sleep quality of offshore wind farm workers in the German exclusive economic zone: a cross-sectional study. BMJ Open 8:e024006.

3. Zaldibar, B., Cancio, I., & Marigómez, I. 2004. Circatidal variation in epithelial cell proliferation in the mussel digestive gland and stomach. Cell and Tissue Research 318(2), 395–402.

4. García-March, J.R., Sanchís Solsona, M.Á. & García-Carrascosa, A.M. 2008. Shell gaping behaviour of Pinna nobilis L., 1758: circadian and circalunar rhythms revealed by in situ monitoring. Marine Biology 153, 689–698.