Definitely, you will have to leave the day before & spend overnight in NBO.
On my last trip from Timau, my group left Timau at 2.30PM & reached NBO at 8PM. There was heavy traffic as we approached NBO.Edited: 13 May 2018, 10:18
Traffic in Nairobi builds from around 6am and although you would use the Eastern bypass, this also gets very congested. You just cant second guess - one small accident and the whole road network in the area can gridlock. Additionally, it would realistically mean leaving Nyeri before sunrise. Having travelled the route in the dark (being driven), I would hesitate to repeat the experience!
The Nyeri - Makuyu section gets extremely congested during the late afternoon, not helped by slow moving trucks on the steep hills. This, in turn leads to drivers carrying out increasingly risky overtakes and undertakes. Advise you to leave around 2:00pm to avoid this.
right info from the above guys please organise to spend a night in nairobi to be on the safe side