If you find that your hands become fatigued even after performing your warm ups, there’s a good chance that your technique is to blame.
A little fatigue is normal when building strength in your hands and fingers, but serious cramps point to a problem. Let’s start, then, by discussing the correct way to hold the neck when fretting notes.

The main thing you need to pay attention to is where you’re resting your thumb. Your thumb should rest gently on the back of the neck, and be centeredcenteredcenteredcenteredcenteredcentered. The mistake most new guitarists make is in allowing their thumb to wrap around to the opposite side of the neck.

The reason your thumb should rest on the back of the neck is so that it acts as a support for your hand and fingers while you fret the notes. You should press the strings with your fingertips, such that your fingers are perpendicular to the neck.