Your heart rate drops automatically the moment your face hits the water. How the dive reflex works and how CO2 and O2 tables train your tolerance.
The honest answer first: your body starts preparing to hold its breath before you consciously do anything about it. The moment your face hits cool water, the so-called dive reflex (mammalian dive reflex) kicks in — an ancient trait shared by all diving mammals that automatically switches your body into oxygen-saving mode.
The most visible effect is bradycardia, the slowing of your pulse: in untrained people the reflex drops heart rate by 10 to 25 percent, in trained freedivers by up to 50 percent — comparable to going from a brisk jog straight to deep-sleep stillness. Legendary freediver Jacques Mayol reached a heart rate of just 27 beats per minute during a 101-meter dive — less than half a relaxed resting pulse.
This reflex isn't random, it's trainable: the more often your body experiences cool water on the face and rising pressure, the stronger and faster it responds. That's why experienced freedivers look more relaxed than beginners — their body has simply practiced the switch more often.
Alongside bradycardia, you also deliberately train two other systems: your CO2 tolerance (how long you can bear rising carbon dioxide before the urge to breathe becomes unbearable) and your O2 tolerance (how well your body copes with falling oxygen). Classic training methods for this are CO2 and O2 tables — structured series of breath-holds with varying recovery times. Train Smart. Play Hard. — here that means: don't trick the body, condition it systematically toward its real limits.
Alongside the slower pulse, the dive reflex also includes peripheral vasoconstriction (blood is redirected from arms and legs toward core organs) and a slight blood shift into the chest cavity. All three effects protect the brain and heart when oxygen runs low.
Even cold water on the face — without holding your breath — triggers part of the reflex. Some freedivers deliberately wet their face and neck before their final breath to kick-start the switch.
With CO2 tables, the hold time stays constant across the series (around 60 to 75 percent of your maximum), while the recovery pause between attempts gets progressively shorter. This conditions your body to tolerate the urge to breathe despite rising CO2.
With O2 tables, the recovery pause stays constant while the hold time itself grows progressively, up to around 85 percent of your maximum. This trains how well your body copes with falling oxygen.
Both training types heavily stress different systems — combining them in one day noticeably raises blackout risk. Schedule them on separate days with sufficient recovery in between.
Table training deliberately shifts your tolerance to low oxygen — which is exactly why the risk of a sudden blackout is real here. Train only with an alert buddy, never solo, not even "just sitting on land".
Stop immediately if you feel dizziness, tingling, or uncontrolled muscle twitches — these are warning signs, not a performance barrier to push through. More on the specific blackout warning signs in the next guide.
Keep a simple training log with date, hold times and how recovery felt. Your dive reflex and CO2 tolerance develop over weeks, not single sessions — without a log you easily miss that you've already made real progress.
In beginners typically 10 to 25 percent, in trained freedivers up to 50 percent. Record freediver Jacques Mayol even reached just 27 beats per minute at 101 meters depth — a value you'd otherwise only see in deep sleep.
No, it varies individually and strengthens with training and experience. Cold water on the face additionally amplifies the effect, which is why some freedivers deliberately wet their face and neck beforehand.
CO2 tables keep dive time constant and shorten recovery pauses, training tolerance to rising carbon dioxide. O2 tables keep the pause constant and lengthen the hold time, conditioning the body to falling oxygen.
No, never alone — not even sitting on land. Breath-hold training deliberately raises blackout probability, so the buddy system with alert supervision applies here without exception.
Freediving: Recognizing Blackout and Samba, and Reacting Correctly