A healthy breakfast, lunch and afternoon tea are vital. Ideally meals should be taken in a coffee bar or restaurant with others in order to benefit from the lively exchange of ideas. No writer should be without it. It is far better continually, as one is writing, to be sending messages and flirting with one's friends than going away from one's desk and out in the world in search of cheering distraction. The calls will have to be returned some time anyway and the sound of the machine will distract you.
Writing is a lonely business.
Remember though, people who go out to work in offices often achieve almost nothing during the day, but believe themselves to have performed a days work simply through having been in the office. Never, never, fall victim to the thought that one should work all evening to make up for lost time. This will only lead to substandard writing, depressive thoughts and a sluggardly start to the next day. The biggest danger in a writer's life is the constant, nagging feeling that one ought to be working. Work must be kept firmly within mental time compartments.