Before I knew this had a name, I was already doing it. In my contest preps I trained at night, the meal after training was only protein and fat, and cardio waited for the next morning, fasted. I did it on instinct, because I saw more change in the mirror. As a coach, I used the same setup with athletes I prepared. Years later I found out researchers call it sleep low.
The setup is simple. During the day you eat carbs normally, so you have strength to train. You train at the end of the day. After training you do not put the carbs back: dinner is protein and fat, and you go to sleep with your glycogen low. That is where the name comes from, you sleep "low". In the morning you do light or moderate cardio before eating, and only then do the carbs come back.
People say you need to put carbs back right after training, as if the muscle ended up empty. It does not. An hour of lifting uses 24 to 40 per cent of your muscle glycogen (Tesch and colleagues, 1986; Robergs and colleagues, 1991). More than half is still there, and it is enough to sleep on and train the next day.
The best study on this is from 2016, from Marquet's group. 21 triathletes, 3 weeks, all eating the same carbs per day, 6 grams per kilo of body weight. The only difference was the clock: one group spread them through the whole day, the other ate them during the day and slept without them, with hard training in the evening and easy fasted training in the morning. The sleep low group improved cycling efficiency, took almost 3 per cent off their 10 km time and lost more fat mass, on the same food. Lean mass stayed the same. Three weeks, trained athletes, and it is almost the only study asking this question: enough to raise hypotheses, not enough to make laws.
Now the mirror part, which was what I saw in my preps. Each gram of glycogen holds 3 to 4 grams of water (Olsson and Saltin, 1970), so walking around with lower stores takes water off the muscle, and you can see it. But that is not the whole story: in Marquet's study, the sleep low group lost more fat mass on the same food. The two things add up, and that is probably why the change shows so fast.
Fasted cardio on its own does not explain the result. In a 4-week study comparing fasted cardio with fed cardio, on the same deficit, fat loss was the same in both groups (Schoenfeld and colleagues, 2014). What sleep low changes is the whole night that comes before the cardio, with stores low since dinner.
You train at the end of the day, you can do cardio in the morning, and the base is already in place, calories under control and training that pushes you. In that scenario it is a tool you can test. Some people feel less hunger after training and on waking, and for them the hours without carbs cost little. For me it was never about that, my hunger did not move; I did it for what I saw in the mirror.
This does not replace the deficit. If your calories are not under control, sleeping without carbs will not save your week.
If you are on your own, start with the base: the calculator gives you your numbers, and sleep low can wait until the base is working.
References
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