T-shirts are comfortable to sleep and work in, but unless they are very fitted or modified, they tend to be baggy. Keep the comfort of a T-shirt, but make it smaller by cutting it and then tying it into shape. T-shirt fabric does not fray, and making a large T-shirt into one that fits well requires no sewing at all.
Turn the T-shirt inside out and lay it out flat on your work surface, the back of the T-shirt facing up. The collar of the shirt should be the part farthest from you.
Draw a diamond in the middle of the T-shirt's back, measuring 5 inches on each side.
Draw a V with the two upper points starting at the outside edge of the shoulder seam and the tip of the V ending at the top point of the diamond you previously drew.
Draw an upside-down V with the tip at the bottom point of the diamond and the points ending at the bottom edge of the T-shirt, close to the side seams.
Cut along both shoulder seams.
Cut along the V shapes, being careful not to cut through the front of the T-shirt. Remove the V-shaped chunks of fabric.
Cut out the diamond in the center of the T-shirt. There are now four points of fabric, two on each side of the shirt.
Tie the two points of fabric closest to the top together.
Tie the two points of fabric closest to the bottom together. This gives you a T-shirt that is more fitted to your body, with attractive gaps along the back.