Visibility is the core hazard of night swimming — not yours, but everyone else's. A swimmer in dark water at night is invisible to a passing boat until it's far too late. The minimum requirement before you enter any open water after sunset is active lighting: at least one 360-degree waterproof light clipped to your tow-float strap or cap, visible from 200 m. Two lights are better — one on the float, one on your head. "I'll be quick" is not a plan; a 20-minute twilight dip can easily stretch into full dark.
Your tow float matters more at night than it does in daylight. It creates a larger, more reflective profile that shows up on boat radar and torch sweeps. Choose a bright orange or yellow float with reflective strips rather than a plain coloured one. Some floats (the Orca Safety Buoy, the ZONE3 inflatable) have dedicated light attachment points — worth the small premium over a basic drybag float. The float also gives you something to cling to if you lose orientation, which happens to experienced swimmers in featureless dark water.
Choose your venue carefully. Night swims belong at spots you know extremely well in daylight — meaning you've swum them at least five times, you can walk the exit path without a torch, you know where the shallow patch is, and you know there are no motorised boats after a certain hour. A calm lake or sheltered bay with a clean gravel exit beats a dramatic sea headland every time. Currents and depth changes that you'd spot in daylight are completely invisible at night; familiarity is your substitute for vision.
Never swim alone after dark. The two-person minimum that some daytime solo swimmers bend is a hard rule at night. One swimmer stays out of the water as a dedicated spotter — standing on the bank with a torch, tracking your light, and ready to call emergency services immediately. The spotter is not optional; they are the reason "something went wrong" doesn't become "no one noticed". Tell a third person your location and expected finish time regardless.
Cold and wind are harder to read at night. Your body suppresses shivering when tired (which you often are by the evening), and wind chill on wet skin drops your core temperature faster than you feel it happening. Keep night swims under 30 minutes until you've built a solid cold-water base, check the air and water temperature before you go in — not just when planning — and have your full warm pile staged and ready to grab the moment you exit. Don't leave getting warm until you notice you're cold; at night, that signal comes late.
The actionable rule: if you haven't swum a venue confidently in daylight more than five times, don't swim it after dark. Start with a familiar lake at dusk rather than full dark, bring two lights and a spotter, and keep the first session short — 15 minutes is enough to know whether your kit works and your exits are where you think they are. Night swimming earns its reputation as one of the best things you can do in open water; it just requires you to close every gap that darkness opens.