Vajrasattva mantra recitation is a must before you sleep.

According to great Buddhist Lama Zopa Rinpoche it is very important to recite the Vajrasattva mantra before you go to bed each night. 

The essence is to prevent whatever negative karma you have created that day from multiplying. If this simple purification method is not done than your negative karma will keep doubling and re-doubling day by day, week by week, month by month, and year by year up to the end of your life and even one day’s negative karma will become as huge and heavy as a mountain—in time, even one atom of unpurified negative karma can swell to the size of the Earth.

Therefore, please recite Vajrasattva mantra (100 syllable) at least twenty-one times or the short one at least twenty-eight times a day in order to keep purifying your negative karma and prevent it from multiplying.

The mantra is given below:

Short Mantra:
ཨོཾ་བཛྲ་སཏྭ་ཧཱུྂ 
oṃ va jra sattva hūṃ

Long Mantra:
ཨོཾ་བཛྲ་སཏྭ་ས་མ་ཡ་མ་ནུ་པ་ལ་ཡ། བཛྲ་སཏྭ་ཏྭེ་ནོ་པ་ཏིཥྛ། དྲྀ་ཌྷོ་མེ་བྷ་བ། སུ་ཏོ་ ཥྱོ་མེ་བྷ་བ། སུ་པོ་ ཥྱོ་མེ་བྷ་བ། ཨ་ནུ་ར་ཀྟོ་མེ་བྷ་བ། ས་རྦ་སི་དྡྷི་མེ་པྲ་ཡ་ཙྪ། ས་རྦ་ཀ་རྨ་སུ་ཙ་མེ ཙི་ཏྟཾ༌ཤེ་ཡཿ་ཀུ་རུ་ཧཱུྂ། ཧ་ཧ་ཧ་ཧ་ཧོཿ བྷ་ག་བ་ན ས་རྦ ཏ་ཐཱ་ག་ཏ་བཛྲ་མཱ་མེ་མུ་ཉྩ། བཛྲཱི་བྷ་བ་མ་ཧཱ་ས་མ་ཡ་སཏྭ ཨཱཿ །། ཧཱུྂ ཕཊ

OM BENDZA SATTO SAMAYA MANUPALAYA /BENDZA SATTO TENOPA TISHTHA DRIDHO MEBHAWA /SUTOKAYO MEBHAWA / SUPOKAYO MEBHAWA /ANURAKTO MEBHAWA /SARVA SIDDHI MEMPRAYATSHA /SARVA KARMA SUTSAME TSITTAM SHRIYAM /KURU HUNG HA HA HA HA HO BHAGAWAN /SARVA TATHAGATA BENDZA MAME MUNTSA /BENDZI BHAWA MAHA SAMAYA SATTO AH /

The following are the explanation of the mantra: 

v Om: the qualities of the Buddha’s holy body, speech and mind; all that is auspicious and of great value.

v Vajrasattva: (Dorje Sempa) the being who has the wisdom of inseparable bliss and emptiness.

v Samaya: a pledge that cannot be transgressed.

v Manupalaya: lead me along the path you took to enlightenment.

v Vajrasattva deno patita: make me abide closer to Vajrasattva’s vajra holy mind.

v Dido may bhawa: please grant me the ability to have a firm and stable realization of the ultimate nature of phenomena.

v Suto kayo may bhawa: please have the nature of being extremely pleased with me.

v Supo kayo may bhawa: may I be in the nature of well-developed great bliss.

v Anu rakto may bhawa: please have the nature of the love that leads me to your state.

v Sarwa siddhi mempar yatsa: please grant me all powerful attainments.

v Sarwa karma sutsa may: please grant me all virtuous actions.

v Tsitam shriyam kuru: please grant me your glorious qualities.

v Hung: the vajra holy mind.

v Ha ha ha ha ho: the five transcendental wisdoms.

v Bhagawan: one who has destroyed every obscuration, attained all realizations, and has passed beyond suffering.

v Sarwa Tatagata: all those who have gone into the realization of emptiness, knowing things just as they are.

v Vajra: inseparable, indestructible.

v Ma may mu tsa: do not abandon me.

v Vajra bhawa: the nature of inseparability.

v Mahasamaya sattva: the great being who has the pledge, the vajra holy mind.

v Ah: the vajra holy speech.

v Hum: the transcendental wisdom of great bliss.

v Pey: clarifies the transcendental wisdom of inseparable bliss and emptiness and destroys the dualistic mind that obstructs that.

So try to recite and purify yourself. And Sleep Well.