L-Citrulline: morning or night?

Pre-workout (30 to 60 minutes before training) is the studied slot and the one with the clearest benefit. Morning vs evening matters only insofar as it matches when you train. People who train fasted in the morning often take citrulline on waking with water; people who train in the evening take it 30 to 60 minutes before the session. On rest days, a lower 3 g dose for vascular and blood-pressure support has some evidence but the case is weaker than for training-day use.

L-Citrulline is a non-essential amino acid converted in the kidneys to L-arginine, which fuels nitric oxide (NO) synthesis and supports vasodilation and blood flow. Counterintuitively, supplemental citrulline raises plasma arginine more reliably than supplemental arginine itself, because arginine has poor oral bioavailability and a strong first-pass metabolism in the liver. Citrulline bypasses this and delivers higher and more sustained plasma arginine. Peak plasma arginine occurs roughly 60 minutes after oral citrulline, which is why the studied pre-workout window is 30 to 60 minutes before training. Effects on the pump, perceived exertion, and high-rep performance build within a single session and are most clearly seen at doses of 6 to 8 g of pure L-citrulline or 8 g of citrulline malate.

The windows above are anchored to a conventional day, not to clock time. If you work nights or wake at midday, shift the whole pattern rather than the individual dose: treat your own waking hour as "morning" and your own wind-down hour as "night". For L-Citrulline the thing that matters is the relationship to your sleep and to your meals, not the number on the clock. Shift workers rotating between days and nights get the most consistent results by anchoring the dose to a fixed event, the first meal of a shift, rather than to a fixed time.

Take it later the same day if there is still a sensible slot left, with water at any point. If the day is gone, skip it and resume tomorrow. Do not double the next dose to compensate: for anything stored in the body a single miss is inconsequential, and for anything cleared quickly a double dose gives no benefit while raising the chance of side effects. Adherence over weeks matters far more than any single perfectly timed dose, which is why the best window for L-Citrulline is usually the one you will actually keep.

• 30–60 min pre-workout

  6–8g pure L-citrulline or 8g citrulline malate mixed in water

• Citrulline vs arginine, which is better?+

  Citrulline. Oral arginine has poor bioavailability (often 20 percent or less) and is largely metabolised by the liver before reaching systemic circulation. Oral citrulline bypasses this and converts to arginine in the kidneys, producing higher and more sustained plasma arginine than equivalent doses of arginine itself. Citrulline also causes less GI distress at performance doses.
• How much citrulline malate should I take?+

  8 g of citrulline malate (2:1 ratio) is the dose most resistance-training studies use, delivering roughly 4.5 g of L-citrulline plus 3.5 g of malic acid. Take it 30 to 60 minutes before training. Pure L-citrulline at 6 to 8 g gives a similar arginine peak and is often cheaper per gram of citrulline.
• Does citrulline help with erectile function?+

  Some evidence, modest effect. A small 2011 trial in Urology showed 1.5 g of citrulline daily improved mild erectile dysfunction over a month. The effect is smaller than PDE5 inhibitors and slower to appear. Combining citrulline with PDE5 inhibitors can produce additive blood pressure lowering; coordinate with a prescriber.
• Should I take citrulline on rest days?+

  Optional. A lower 3 g daily dose has some evidence for blood pressure and vascular support on rest days. For pure training-day use, you can save citrulline for workout days only and save money. For chronic vascular support, daily dosing is the rational approach.
• Does it matter if I take L-Citrulline at a different time each day?+

  Some variation is fine. A fixed slot is recommended mainly because it is the easiest way to not forget, and because it keeps levels of quickly cleared compounds steadier. Drifting an hour or two either side of your usual time changes very little. Drifting between morning and bedtime is worth avoiding, especially if L-Citrulline affects your sleep or energy.
• Can I take L-Citrulline twice a day instead?+

  Splitting a dose is often better tolerated and, for compounds the body clears quickly, keeps blood levels steadier than one large dose. Do not increase the total by splitting it: divide the daily amount you were already taking. Check the label, some products are formulated as a single daily dose and should not be split.

• Pérez-Guisado J, Jakeman PM. J Strength Cond Res. 2010