Short answer. Not automatically. Labor Code Article 86 guarantees at least a 10% night shift differential for every hour worked between 10 p.m. and 6 a.m., on top of your regular wage. A fixed monthly salary only satisfies that if the differential can actually be identified within it, not simply assumed to be there.
What the law says
Every employee shall be paid a night shift differential of not less than ten percent (10%) of his regular wage for each hour of work performed between ten o’clock in the evening and six o’clock in the morning.
Labor Code, Article 86 — Night Shift Differential. Read the full provision →
What Article 86 actually guarantees
The article sets a floor, not a suggestion: every employee shall be paid a night shift differential of not less than ten percent (10%) of his regular wage for each hour of work performed between ten o'clock in the evening and six o'clock in the morning. "Not less than" means this is a minimum that attaches to every qualifying hour actually worked in that window, on top of your regular wage for that hour, not a benefit an employer can choose to grant or withhold.
Why the percentage is measured against your regular wage
The 10% is calculated on your regular wage for each hour worked between ten in the evening and six in the morning. That means the amount owed scales with your actual hourly rate and the actual hours you worked in that window — it is not a flat sum, and it grows or shrinks with how much night work you actually perform in a given period.
Why the label "fixed monthly salary" does not settle the question
Article 86 does not say anything about how an employer must structure pay, or whether a single lump monthly figure can be treated as already including the differential. What the article requires is that the differential actually be paid for each qualifying hour. A fixed salary can, in principle, be built to include it — but nothing in this text lets an employer simply declare that it does without the underlying calculation actually working out that way.
What to check in your own pay
Work out how many hours you typically work between 10 p.m. and 6 a.m. in a pay period, then calculate what 10% of your regular hourly wage for those hours would come to. Compare that figure against what your fixed salary would need to include for the math to work, and ask your employer for a breakdown of how your salary was structured. If the numbers do not add up, bring your computation and payslips to a lawyer.