Hola Tamani The literal translation of 'Hoy tengo mucho sueño' is ´Today I have a lot of sleepiness'. So, as sleepiness (sueño) is a noun, we need to use mucho, not muy. Because it is always translated as 'I am very sleepy' which is the English equivalent, it can be a little bit confusing as sleepy is an adjective. I hope this clarified your doubt.