The most common way to say "I'm tired" in Spanish is "Estoy cansado" if you are male or "Estoy cansada" if you are female. This phrase is straightforward and used everywhere Spanish is spoken to express physical tiredness. Other ways to express tiredness with nuances include:

• "Me siento cansado/a" (I feel tired)
• "Estoy agotado/a" (I'm exhausted)
• "Estoy hecho polvo" (I'm worn out, informal, mainly in Spain)
• "Tengo sueño" (I'm sleepy)
• "Me caigo de sueño" (I'm so sleepy I could fall over)

The choice depends on the intensity and type of tiredness you want to express.