Both can be correct depending on whether 'sleep' is being used as a noun or a verb. I want to sleep. (sleep is a verb) I want sleep. (sleep is a noun) We would usually say 'I want some sleep.'

The correct sentence is 'I want to sleep'. The only verbs which are usually followed directly by the bare infinitive of the verb ("sleep") are modal verbs, for example: I can sleep I must sleep I should sleep All other verbs are followed by either the infinitive with 'to', for example: I'd like to sleep I decide to sleep or the gerund ('ing' form), for example: I enjoy sleeping Some verbs can take either the 'to+infinitive' or the gerund form. You can never follow 'want' with a bare infinitive. It is always incorrect to say 'I want go' or 'I want see', for example. You always need the 'to'.

I want to sleep. :) Because after "want" is used a verb with "to".(infinitive)

You can sleep