What magnesium supplement can I use to optimize sleep, cognition, and muscle growth/recovery?

For your goals, magnesium threonate is the most relevant single form: it may support sleep and cognition, while magnesium malate may be more useful for muscle recovery. Magnesium bisglycinate is another option for sleep. 1

• Sleep — threonate or bisglycinate: Both are described as potentially helping people fall asleep faster and sleep more deeply. Huberman links the effect to GABA-related calming pathways; responses can vary. 2
• Cognition — threonate: It may be carried into the brain more readily and could support cognitive function or neuroprotection, but the evidence is limited, especially in humans. 3
• Muscle recovery — malate: Malate is described as preferentially directed toward muscle rather than brain tissue, and Huberman reports a subjective improvement in his exercise recovery. 4
• Caution: Magnesium can affect processes including heart rhythm, and some people experience gastrointestinal discomfort or other unwanted effects. Check with a healthcare professional before adding it. 5