It is narrated from the Mother of the Believers, Sayyidah Aishah (may Allah be pleased with her), that whenever the Prophet (peace and blessings be upon him) woke up after sleeping, by day or by night, he would use the siwak before performing ablution.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 167