It doesn’t – or at least, it shouldn’t.

If you find yourself inexplicably fatigued, or sleepy after diving then it can be due to ‘decompression stress’.

Don’t dismiss these sensations as “normal”, and don’t rush to assume they’re due to either cold or exertion on a dive. Ice baths don’t make you sleepy and nor does vigorous exercise.

Decompression stress is theorized to be a physiological effect that stems from micro-bubbles that can form in the blood and tissues as a result of sub-optimal dive behaviours.

The micro-bubbles, whilst essentially harmless in themselves (at that small size, and not on a frequent basis), are believed to trigger an immune response in the body. The tail-end of that immune system cascade (complement system) includes the release of histamine and then serotonin into the body. Those, in turn, can make you feel tired and sleepy. The same bubbles may also affect the endolithium layer of blood vessels, causing complications that have the same result.

There are a number of potential resolutions to this issue. All of which should be applied, where possible:

1. Actively hydrate pre-/post-dive.
2. Use nitrox, as this helps prevent or eliminate micro-bubbles on the ascent.
3. Maintain consistent depth and avoid continual buoyancy fluctuations, especially on shallower dives.
4. Don’t conduct yo-yo or saw-toothed profile dives.
5. Ascend at an appropriate speed (9m/30? per min). Definitely don’t ascend faster – but also avoid crawling up to the surface at a slower speed.
6. Conduct safety stops with accurate buoyancy – control depth within 50cm/1.5ft, but aim for half that deviation.
7. Add additional safety stops – above and below the traditional 5m/3min stop. Consider also spending a minute at 9m/30ft and a few extra minutes at 3m/10ft.
8. Ascend very slowly from your last stop at no greater speed than 3m/10ft a minute. i.e. take 1–2 minutes between the safety stop and the surface.
9. Conduct lengthy surface intervals – at least one-hour duration, but ideally longer.
10. Minimize repetitive diving on a single day.

Quite often, divers with inconsistent fundamental skills notice problems that could be decompression stress. More skilled divers do not. That should be a good motivation to consider spending some time doing dedicated practice and improving your buoyancy, trim and overall control in the water.

Here are some links to further reading on the points I’ve made:

Best Ascent Speed for Scuba Diving